To determine whether the maternal metabolic control and/or the use of hypoglycemic drugs during early gestation is associated with a risk of congenital malformations, beginning on January 1989 to December 1994, clinical data from 16 Italian centers were collected retrospectively and entered in a computerized data base: 517 pregnant women with pregestational diabetes mellitus, 362 with insulin-dependent diabetes mellitus (IDDM) (mean age 28.13 +/- 4.8 years), 130 with non insulin-dependent diabetes mellitus (NIDDM) (mean age 33.01 +/- 5.32 years) and 25 with impaired glucose tolerance (IGT) (mean age 32.48 +/- 6.2 years). The percentage of congenital malformations in NIDDM that took oral hypoglycemic drugs was 11.6% respect to 1.4% of NIDDM that did not take hypoglycemic drugs (p < 0.01) and 3.7% of IDDM. Fasting blood glucose, glycosylated hemoglobin and urine keton bodies were more elevated in IDDM respect to NIDDM (p < 0.005). The percentage of malformations in offspring of NIDDM mothers is higher with respect to that of IDDM women, in spite of a better metabolic control.