We retrospectively reviewed the records of 56,919 infants born from 1966 through 1986 to determine the incidence, significance, and possible associations of single umbilical artery (SUA). One hundred fifty-nine infants were affected. The sex distribution was approximately equal. Fourteen infants (8.8%) were twins who were discordant for SUA. The smallest twin was the one affected in all but two cases. Of the 159 infants, 16 were stillborn and 19 died neonatally. The mean birth weight of all infants with SUA was 2.47 kg, and the mean gestational age was 35.9 weeks. Young primipara and older multipara mothers were more commonly affected. There was an increased incidence of SUA associated with maternal diabetes, epilepsy, toxemia, antepartum hemorrhage, hydramnios, and oligohydramnios. Twenty-six (16.4%) of the associated placentas had anomalies. Seventy-one infants (44.7%) had other congenital malformations. The presence of these malformations was associated with increased mortality. Screening for renal anomalies with ultrasonography or intravenous pyelography was performed in 27 patients and yielded positive results in five (18.5%). We recommend that renal ultrasonography be performed on all infants with SUA.