A study was performed of congenital malformations in infants whose mothers used nonsteroidal anti-inflammatory drugs (NSAIDs) in early pregnancy. Data were obtained from an ongoing prospective recording of drug use during the first trimester. During the period July 1, 1995 through December 31, 1998, 2557 infants were born to women who reported the use of NSAIDs in early pregnancy. The OR (after consideration of maternal age, parity, and smoking habits) for any congenital malformation was 1.04 (95%CI 0.84-1.29), but the OR for cardiac defects reported to the Medical Birth Registry was 1.86 (1.32-2.62) based on 36 instances, and for orofacial clefts 2.61 (1.01-6.78) based on only six instances. By using other information sources, another four infants with cardiac defects were identified. There was no drug specificity for cardiac defects but among six mothers of infants with orofacial clefts, five had used naproxen.