Four cases of myasthenia gravis in early childhood are reported. One case is congenital, and exhibited no fetal movement, suggesting that the myasthenia had an intrauterine onset. The relationship of the myasthenia to the coexisting maternal hyperthyroidism—the first case reported—is considered in view of an apparent relationship in adults. That other "toxic" states may play a part is possible. One of the cases had a sister and possibly other siblings affected with myasthenia gravis. From the published accounts it appears that a familial incidence of myasthenia is considerably higher in childhood. It is, however, concluded that this does not represent a genetic factor—rather that unknown environmental factors are operative. Two cases showed severe exacerbations during intercurrent infections—one being in complete remission between infections. The manner in which an infection results in an exacerbation is not clear. The myasthenic-like state affecting the children of myasthenic mothers is discussed in relation to myasthenia. It is concluded from the evidence at present available that the defect in myoneural transmission in myasthenia gravis is the effect of a circulating substance whose nature, source and mode of action, remains obscure; but appears to be continuously or intermittently secreted. While myasthenia gravis is a rare disorder, it is becoming increasingly apparent that the incidence in early childhood is considerably higher than previously suspected. Some of the reasons causing difficulty in diagnosis are discussed.