In a prospective study of 100 consecutive diabetic pregnancies, 1 was complicated by hyperthyroidism and 4 by hypothyroidism. The association of hypothyroidism with diabetes mellitus merits special attention as this combination of diseases affected 4 of 20 (20%) White's class D and F diabetics. Hydramnios and/or spontaneous premature labor were features of diabetic pregnancies complicated by compensated hypothyroidism. Delayed diagnosis of hypothyroidism may have contributed to the fatal congenital malformations in one of these fetuses, but the remaining infants survived, including 1 infant suffering from severe respiratory distress syndrome (RDS).