We report observations made on 149 patients with juvenile myasthenia gravis studied from onset of disease for as long as 40 years. Median follow-up was 17 years; minimum was 4 years. Eight other patients with congenital myasthenia gravis were studied separately. Of the juvenile myasthenic patients, 85 (57%) underwent thymectomy because of diseases severity. In juvenile myasthenia gravis, a spontaneous remission rate of 22.4 per 1,000 person-years was observed, regardless of disease duration. A remission rate of 260 per 1,000 person-years was seen during the first year after thymectomy, with a rate of 95 per 1,000 person-years during the next 2 years. Early surgery, presence of bulbar symptoms, absence of ocular signs or generalized symptoms, onset of symptoms between ages 12 and 16, and presence of other immune disease were associated with increased postoperative remission rates. Epilepsy (4 patients) and neoplasia (7 patients) were the most frequent associated nonimmune disorders; rheumatoid arthritis (5 patients), juvenile-onset diabetes mellitus (3 patients), asthma (3 patients), and thyroid disease (3 patients) were the most frequent associated immune diseases.