Previous studies have shown that diabetic women more commonly have complications of pregnancy and adverse infant outcomes than do other women. However, most of the studies have not evaluated women with gestational diabetes separately. The purpose of this study was to evaluate pregnancy complications and infant morbidity and mortality among births to women with gestational diabetes and women with established diabetes. Birth certificate data from 1984 in Washington State linked with death certificate data provided information on complications of pregnancy and infant outcome for 422 gestational diabetics and 144 established diabetics. A comparison group of 856 non-diabetic women who delivered a child was selected at random. Both established and gestational diabetic women were more likely to be reported to develop pre-eclampsia (relative risk (RR) = 4.0 and 9.6). Established and gestational diabetic women were also at increased risk of delivery by Caesarean section (RR = 2.1 and 5.0). Infants of established diabetics had a higher risk of congenital anomalies (RR = 7.6) than infants of non-diabetics and were at increased risk of death in the first 4 weeks (RR = 7.9) and the first year of life (RR = 5.0). Gestational diabetics were more likely to have high birthweight babies (greater than 4000 g) (RR = 2.1) while established diabetics were more likely to have babies at either extreme of birthweight (greater than 4000 g, RR = 1.7; less than 2500 g, RR = 3.2). We conclude that both gestational and established diabetes are associated with important increases in risk of pregnancy complications and adverse infant outcomes.