Mary Sue Estes appeals the denial of her application for disability benefits under provisions of the Railroad Retirement Act of 1974 (RRA) authorizing payment of an annuity to the child of a deceased employee covered by the RRA if the child was disabled before reaching the age of twenty-two. 1 The Railroad Retirement Board (RRB) denied Estes’ application on the ground that she was not disabled before she was twenty-two. We reverse.

*1437 BACKGROUND

Estes, the child of a covered employee, was born on March 22, 1955. She currently suffers from chronic multiple sclerosis and is confined to a wheelchair. A review of the facts is necessary to determine whether Estes was disabled when she reached her twenty-second birthday on March 22, 1977.

The medical evidence establishes that Estes was afflicted with multiple sclerosis by 1973. When it was first diagnosed in 1976, the disease had already caused Estes to suffer severe symptoms: foot-drop, visual problems, sensory defects in the lower extremities, and abnormal fatigue. In July of 1976, Estes was hospitalized and treated with adrenocorticotrophic hormone (ACTH); her symptoms improved slightly.

She attended school that summer but because of her physical condition was only able to take afternoon classes. In the fall of 1976, she obtained work as a clerk at a ear dealership. Again, her illness severely hindered her efforts; she was easily fatigued and neglected her work. Estes testified that she was able to continue working only because a sympathetic supervisor assumed some of her tasks.

After Estes turned twenty-two her condition worsened. She was again hospitalized in July of 1977 with persistent numbness in her right hand and foot as well as pain in moving her right eye. Her symptoms were again treated with ACTH. She thereafter left her job at the car dealership, and since then has only briefly held two part-time jobs. She attended college intermittently until 1979.

As is generally true of multiple sclerosis victims, Estes’ progressively disabling condition was characterized by periods of remission and exacerbation. Between 1976 and 1980, Estes suffered numerous symptoms of varying severity. In periods of remission some of her symptoms would regress or even disappear. Her condition, however, deteriorated markedly in the first half of 1980 and in April of that year she was deemed disabled by the Social Security Administration when she applied for Social Security disability benefits. 2

Estes then applied for a disabled child’s annuity under the RRA on September 8, 1981. The application was initially denied by the Bureau of Retirement Claims and later upheld on reconsideration by the Bureau. The denial was sustained at a hearing before an appeals referee, who determined that Estes was not “disabled” until May of 1980. This decision was affirmed by the RRB over a strong dissent. Estes then appealed to this court.

DISCUSSION

A. Standard of Review

This Court will not set aside a decision of the RRB “if it is supported by substantial evidence, is not arbitrary and has a reasonable basis in law.” Akins v. Railroad Retirement Board, 721 F.2d 652, 653 (9th Cir.1983); Lowe v. Railroad Retirement Board, 294 F.2d 115, 116 (9th Cir.1961) (per curiam).

B. Eligibility

At the outset we address the proper standard for determining Estes’ eligibility for benefits under the RRA. 45 U.S.C. § 231a(d)(l)(iii) entitles the child of a covered employee to benefits if the child is “under a disability which began before he attained the age of twenty-two.” Under the RRB regulations, a person is under a “disability” if he or she is unable to regularly perform “the substantial and material duties of any regular and gainful employment.” 20 C.F.R. § 208.17 (a). 3 According *1438 ly, we must determine whether Estes’ condition prevented her from regularly engaging in substantial gainful employment prior to March 22, 1977.

C. Onset of the Disability

In applying the RRA disability standard to the facts of this case, we note that its interpretation is before us for the first time. Language identical to the RRA standard, however, appears in the disability provisions of the Social Security Act (SSA). 42 U.S.C. § 402 (d)(1)(B) (applicant must be “under a disability ... which began before he attained the age of 22”). Similarly, the SSA standard for disability is whether the applicant can “engage in any substantial gainful activity____” 42 U.S.C. § 423 (d). Accordingly, the RRB itself uses the Social Security Administration’s regulations in determining disability under the RRA, and SSA cases are persuasive precedent in RRA disability cases. See, e.g., Goodwin v. Railroad Retirement Board, 546 F.2d 1169, 1172 (5th Cir.1977); Parker v. Railroad Retirement Board, 441 F.2d 460 , 463 n. 6 (7th Cir.1971); Duncan v. Railroad Retirement Board, 375 F.2d 915, 917-18 (4th Cir.1967). In particular, we find the Sixth Circuit’s well-reasoned application of the SSA standard to a multiple sclerosis victim in Parish v. Califano, 642 F.2d 188 (6th Cir.1981), directly applicable to the facts in this case.

There is no dispute that Estes is now disabled by multiple sclerosis and that the condition existed before she was twenty-two. The RRB contends, however, that the condition did not become disabling until after Estes reached twenty-two; that is, she was able to engage in substantial gainful employment up to March 22, 1977.

The record indicates that before Estes was twenty-two she had experienced foot-drop, sensory problems in both lower extremities, visual problems in her left eye, and abnormal fatigue. While she worked at a car dealership for several months prior to her twenty-second birthday, her illness rendered her job performance inadequate. The appeals referee, while noting that healthy activity occurred “[i]n the periods of remission of her disease,” nevertheless found that the condition had not become disabling until 1980.

This conclusion is untenable in light of Parish , which is the only case under the RRA or SSA applying an age requirement to a progressively disabling disease like multiple sclerosis. The question before the Parish court was whether normal activity in a period of remission established “substantial gainful activity” under the SSA disability standard. Like Estes, the petitioner in Parish suffered from multiple sclerosis before reaching twenty-two but was able to work and attend school in periods of remission. Parish, 642 F.2d at 190-92 . The Social Security Administration argued that such activity after reaching the age of twenty-two indicated that the petitioner in Parish was not disabled until after she reached her twenty-second birthday. The administrative judge in Parish , like the referee in this case, relied on the petitioner’s employment in finding that the disease had not prevented substantial gainful activity by the petitioner. Id. at 192-93 .

The Sixth Circuit reversed, holding that “[t]he fact that plaintiff worked ... is not necessarily substantial evidence of substantial gainful activity.” Id. at 192 . The Sixth Circuit properly observed that multiple sclerosis can be disabling notwithstanding normal activity in periods of remission. The court said:

Parish , at 193 (emphasis added).

We find the instant case essentially indistinguishable from Parish . Estes’ employment during 1976 and early 1977 most like *1439 ly occurred in a remission period. She had previously suffered severe symptoms, and in July of 1977 was again hospitalized as a result of even more serious disabilities. Under the logic of Parish , it appears that Estes was suffering from a progressive disability by 1976, her twenty-first year. That the next year was a period of remission, rather than exacerbation, does not alter the fact that she was indeed suffering from a disability. The fact that a disabled multiple sclerosis victim was employed during a remission period, especially where the victim’s performance was inadequate, does not establish substantial gainful employment. Parish , at 191-92; see also Goodwin v. Railroad Retirement Board, 546 F.2d 1169, 1171 (5th Cir.1977) (applicant disabled despite employment where job performance was inadequate and environment was sheltered and sympathetic).

CONCLUSION

Neither the appeals referee nor the RRB applied the principles enunciated in Parish to the facts of this case. Their reliance on Estes’ activity in periods of remission as establishing substantial gainful employment is misplaced. The record, viewed in light of the sound principles elaborated in Parish , establishes that Estes was disabled by multiple sclerosis by March 22, 1977.

Accordingly, we reverse the decision below and remand to the Bureau of Retirement Claims so that they may determine the amount of benefits to which Estes is entitled.

REVERSED and REMANDED.

. The RRA provides in pertinent part:

45 U.S.C. § 231a(d)(l)(iii) (emphasis added).

. Estes is currently receiving Social Security benefits. It is unclear from the record whether the Social Security benefits Estes now receives are supplemental security income or disability benefits based on her own earnings record.

. This section provides: