The capacity of a general ultrasound screening program to detect fetal malformations affecting the urinary tract was evaluated in an epidemiologic study. A total of 11,986 pregnant women, representing 97% of the pregnant population in Malmö, Sweden, from April 1978 through August 1983 were examined. On routine examinations performed by midwives in the 17th and 33rd gestational weeks, the fetal anatomy was carefully surveyed. The overall frequency of fetal malformations was 0.5%, with urinary tract abnormalities representing approximately 50% of the total number. In 20 of 33 cases, the ultrasound findings were those of hydronephrosis or hydroureter. In ten cases, a cystic renal malformation was found, and the remaining three cases represented double renal pelvis, Potter syndrome, and posterior urethral valve. A total of 28 abnormalities were unilateral and five bilateral. No case of unilateral absence of renal tissue was noted prenatally. Prenatal diagnosis of urinary tract abnormalities known to precipitate neonatal urosepsis and subsequent renal scarring and other complications makes it possible to start an early antibiotic prophylaxis regimen. A complete workup of the infants can be started early and before life-threatening complications occur.