We assessed the influence of race, sex, and puberty upon clinical features and outcome in 115 patients with autoimmune juvenile myasthenia gravis (JMG). These demographic variables influenced not only disease incidence but also disease severity, response to therapy, and outcome, despite comparable therapeutic strategies. Among white patients, those with prepubertal onset had low incidence and equal sex ratio; the incidence in females increased during and after puberty; males had lesser disease severity than females during and after puberty (p < 0.05); spontaneous remissions were most frequent (44%, p = 0.001) and persistence of active JMG for more than 10 years was least frequent (p = 0.05) in patients with prepubertal onset; remissions were more frequent after early than late thymectomy (p = 0.03); and final disease severity was less after early than late thymectomy. Black patients had similar incidence, disease severity, and sex ratio (F:M = 2:1) with pre-, peri-, or postpubertal disease onset; infrequent spontaneous or treatment-induced remissions; and the same final disease severity after early or late thymectomy. These observations imply that race and sex hormones modify the clinical features and outcome of JMG; spontaneous remissions are common in white patients with prepubertal disease onset; early thymectomy may be more beneficial than late thymectomy in white patients; and the role of thymectomy in the youngest patients is uncertain. We suggest that demographic factors should be considered when evaluating past and future therapeutic strategies for JMG.