Forty-two pregnancies in 39 myasthenic mothers were studied between 1978 and 1987. In 4 cases myasthenia gravis began during pregnancy and for 20 patients the clinical condition exacerbated in 15 cases, usually during the first 3 months, or during the postpartum. Except for 1 case, clinical exacerbation was controlled by anticholinesterase drug adjustment. Obstetrical problems were uncommon: abortion in 1 case and premature delivery in 4 cases. In 2 babies with severe fetal involvement polyhydramnios was present. Twenty-one babies had neonatal myasthenia gravis (NMG). In 17 cases, transient symptoms were present at delivery or shortly afterwards and full recovery occurred from a few days to 4 months. In 4 babies the clinical presentation was atypical: 1 showed a long evolution (15 months) and residual facial bilateral weakness, and three others presented fetal involvement (arthrogryposis). In these latter cases, presentation was severe with polyhydramnios, respiratory failure and long standing evolution in 2 cases (4 months and 1 year). Antiacetylcholine receptor antibodies (anti-AChR Ab) were found in all myasthenic babies and in 19 out of 20 asymptomatic babies. Maternal antibody titers were usually slightly higher than umbilical cord titers. There was a good correlation between maternal titer and onset or severity of disorder in baby. Among the 15 high titer mothers (greater than 60 nM), 13 had a myasthenic baby, 6 of them with serious disease. Conversely all low titer mothers (less than 10 nM) had a symptom-free baby. Therefore, anti-AChR Ab titration in the mother is predictive for NMG onset. Mother's myasthenia gravis severity and treatment were not correlated to the clinical condition of the newborns.