Of 51 patients with a diagnosis of myasthenia gravis made during infancy or childhood, 10 had transient symptoms in the neonatal period, 6 had symptoms at birth which persisted, and 35 had persistent symptoms which began after the first year of life. The neonatal transient type of myasthenia gravis occurred in infants whose mothers were affected with the disease; 1 of 4 infants untreated with Prostigmin (neostigmine) died. The neonatal persistent (congenital) type of myasthenia gravis occurred in infants born to unaffected mothers. Symptoms were mild but somewhat refractory to drug therapy, and the course of the disease was protracted. The juvenile type of myasthenia gravis presented after 10 years of age in 74% of the patients, the majority of whom were girls. Ptosis and weakness of limbs were the commonest presenting symptoms. The illness was frequently mis-diagnosed as hysteria. Thymectomy was performed in 21 patients; complete remission was obtained in 6 (29%) patients, and objective improvement occurred in 12 (57%). Of 14 patients treated only with cholinergic drugs, 2 (14%) recovered completely and 11 (79%) were objectively improved; 1 patient of this series died of the disease. Complete remission of symptoms followed irradiation of the thymus in 1 young child.