The neurodevelopmental consequences of maternal insulin-dependent diabetes were studied in 109 infants of diabetic mothers and 90 control infants. The infants born to diabetic mothers included 70 "early entry" subjects and 39 "late entry" subjects. Maternal diabetes control during pregnancy was significantly better in "early entry" mothers than in late-entry mothers, as determined by glycosylated hemoglobin levels. Infants were examined by a psychologist and a developmental pediatrician unaware of group status at 6, 12, 24, and 36 months of age; 71% of the subjects completed the 3-year-study. Neurodevelopment of early-entry subjects was similar to that of control subjects, whereas late-entry subjects scored less well on language measures. Mean head size in late-entry subjects was significantly less (p = 0.03) than in either control subjects or early-entry subjects at age 3 years, and correlated negatively with glycosylated hemoglobin levels during all three trimesters. Less optimal intellectual development was associated with reduced head circumference. In addition, the presence of major congenital malformations was associated with reduced developmental performance through age 2 years. Our results indicate that mothers with insulin-dependent diabetes who maintain good control during pregnancy can expect to have infants who are neurodevelopmentally normal; mothers whose diabetes is less well controlled may have infants with less optimal neurodevelopment.