Multiple sclerosis and myasthenia gravis are the most common autoimmune diseases affecting the nervous system, with generally a female predominance in adults. To determine the gender distribution in childhood, we reviewed 28 patients with definite multiple sclerosis and 44 patients with myasthenia gravis with onset before 16 years. In myasthenia gravis, a significant female preponderance was observed only with onset after 10 years of age; earlier disease showed no gender difference. Multiple sclerosis tended to affect boys more than girls before age 10 years and both sexes equally after this age. Gender prevalence in childhood multiple sclerosis and myasthenia gravis is different than in adult series; these results may provide insight into pathogenesis or clinical approach.