We investigated the course and outcome of pregnancy and its influence on muscle weakness in 12 females with proximal spinal muscular atrophy (SMA) who delivered a total of 17 infants when aged 18-32 years. In 4 females the SMA clearly followed an autosomal recessive mode of inheritance. The disease was autosomal dominantly inherited in 2 patients; the other 6 were sporadic cases. Ages of onset of SMA ranged from 8 months to 29 years; all the females learned to walk, and 10 out of 12 are still ambulatory aged 30-60 years. Pregnancy and delivery were complicated in 10 out of 12 patients by premature labour (4), prolonged labour (3) and delayed postpartum recovery (6). Caesarean section was performed in 3 cases. No deleterious effects on fetal outcome could be detected. Exacerbation of muscle weakness after the second trimester of pregnancy was experienced by 8 females: 5 noticed a persistent deterioration of SMA; in 3 muscle weakness worsened temporarily during pregnancy and was followed by marked improvement in the puerperium. The psychological perceptions, in retrospect, of 10 females concerning their decision to have children were evaluated.