We analyzed relationships among pubertal stage at disease onset, sex, disease severity, and acetylcholine receptor antibody (AChR Ab) levels in 46 patients with autoimmune juvenile myasthenia gravis (JMG). Female predominance was least in children with prepubertal disease onset (F:M = 1.3:1) and increased in patients with peripubertal (F:M = 1.8:1) and postpubertal (F:M = 14:1) onset. Seronegative JMG was most common in children with early disease onset: 4 of 9 (44%) with prepubertal, 4 of 22 (18%) with peripubertal, and 0 of 15 (0%) with postpubertal onset were seronegative. The rapid therapeutic response to plasmapheresis was useful in distinguishing some patients with seronegative JMG from those with congenital myasthenia gravis (CMG). The high frequency of seronegative JMG in patients with prepubertal onset indicates that AChR Ab assays do not adequately discriminate between JMG and CMG in young children. Furthermore, the different sex distribution in patients with different pubertal stages at disease onset suggests that sex hormones play an important modulating role in JMG.