The purpose of this study is to investigate the risks of delivery of malformed infants in diabetic mothers in comparison with those in non-diabetic mothers and to clarify risk factors for malformation due to maternal diabetes. The delivery records available at the Department of Obstetrics and Gynecology, Mie University School of Medicine between 1979 and 1992 showed 103 mothers with diabetes mellitus among 4,353 pregnancies. The incidence of malformation in infants born to diabetic mothers was 9.7%, and it was 2.5% overall. By logistic regression analysis of all mothers in this series, diabetes as well as maternal age, and fetal age were shown to be significant risk factors for malformation. A further logistic model for diabetic mothers revealed that a younger age of the mother at the time of labour, low fetal age, older age at onset of diabetes, previous history of pregnancies, and an Hemoglobin A1 (HbA1) level higher than 9% were independent risk factors, among which HbA1 (>9%) showed the highest odds ratio of 24. Careful management is therefore needed to reduce the risk of malformation in pregnancy of diabetic mothers.