The purpose of this study was to evaluate the accuracy of antenatal ultrasonography in the detection of fetal anomalies among twin gestations cared for in a specialized antepartum twin clinic. A retrospective study was performed of 245 consecutive twin gestations followed in our twin clinic. The sensitivity, specificity and diagnostic indices of ultrasound to detect prenatal anomalies in the 490 infants delivered from July 1988 to October 1994 were determined. Fourteen infants had isolated congenital anomalies, and ten had multiple anomalies. The overall prevalence of congenital anomalies was 4.9%. Antepartum ultrasound examination had a sensitivity of 88% and a specificity of 100% for the detection of an anomalous infant, with a positive predictive value of 100% and a negative predictive value of 99%. The sensitivity of ultrasound for each individual anomaly within the cohort was 82%, with 100% specificity, 100% positive predictive value, and 98% negative predictive value. Our conclusions are that serial antepartum ultrasound examination of twins for congenital anomalies in our institution is very specific, with high positive and negative predictive values. The sensitivity, while also excellent, has limitations, which should be discussed with the patient.