We conducted a systematic search of the world literature up to January 1, 1990 on the use of angiotensin-converting enzyme inhibitors for treatment of hypertension during pregnancy. A total of 25 publications reported 85 pregnancies in 81 women, including three twin pregnancies. Captopril had been used in 49, enalapril in 35, and both drugs in one of these pregnancies. The number of unbiased data are too limited to permit firm conclusions on teratogenicity, if any, of these agents. Contrary to earlier suggestions, we found no evidence that the use of these agents increases the likelihood of low weight for gestational age, respiratory distress syndrome, and/or persistent ductus arteriosus. Use of these agents in pregnancy can cause severe disturbance of fetal and neonatal renal function, such as oligohydramnios, pulmonary hypoplasia, and long-lasting neonatal anuria. Although the true incidence of these perinatal problems cannot be derived from the type of data hitherto available, there are strong suggestions that renal dysfunction is more common with the use of enalapril than with captopril. The frequency of serious perinatal complications is high enough to warrant extreme reluctance in prescribing angiotensin-converting enzyme inhibitors during pregnancy.