Using data from the Spanish Collaborative Study of Congenital Malformations (ECEMC), consisting of 19,039 consecutive malformed infants of unselected pregnancies, we have analyzed the relationship between nongestational maternal diabetes and different groups of congenital anomalies. The total sample of malformed babies was separated into two groups: children of nongestational diabetic mothers, and those of nondiabetic women. Analysis of the proportion of children identified in each group with different types of anomalies allows us to demonstrate that the most characteristic group of congenital anomalies observed in the children of diabetic women belongs to the caudal dysgenesis complex, while congenital heart defects are the most frequent malformations in these children. On the other hand, these children also present a multiple congenital anomalies (MCA) pattern more frequently than those of nondiabetic women. From this study it is also clear that the proportion of blastogenic malformations is higher in the offspring of diabetic mothers. © 1994 Wiley-Liss, Inc.