We assessed the factors influencing the birth weight of infants born to 83 women with insulin-dependent diabetes mellitus (IDDM) over a 5-yr period. Maternal glycosylated hemoglobin (HbA1) concentrations at delivery correlated with the percentile birth-weight ratios (r = .43, P < .001) and indicated that ∼18% of variance in the birth weight could be ascribed to glycemic control in the third trimester. Fetal macrosomia occurred in 22 (27%) pregnancies. When 20 of these pregnancies were compared closely with 20 nonmacrosomic pregnancies in diabetic women, the mothers of macrosomic infants were found to be more obese, have a history of previous macrosomic birth, and have higher concentrations of serum human placental lactogen and urinary estriols in the third trimester. Macrosomic pregnancy was further distinguished by accelerated fetal growth (judged by serial ultrasonography) from the 32nd wk of gestation and by biochemical (but asymptomatic) hypoglycemia in the neonate. In our study, no serious neonatal morbidity could be attributed to macrosomic pregnancy. Good glycemic control was attained in both groups, and no significant differences between the groups in overall glycemic control throughout pregnancy were noted. Thus, despite good glycemic control, macrosomia remains comparatively common in modern pregnancy complicated by IDDM, and factors other than maternal hyperglycemia must contribute to its etiology.