Sex determination in both fetuses and infants with ambiguous external genitalia usually necessitates time-consuming and costly karyotyping. We propose a simple, rapid, and reliable method of prenatal and postnatal sex determination by means of the PCR, a technique currently used to identify gender for forensic purposes. DNA was extracted from 20 samples of whole blood from infants with ambiguous genitalia and from five samples of amniotic fluid. Three markers were amplified from each specimen: a Y chromosome alphoid repeat sequence and two homologous genes, amelogenin (AME) and zinc finger protein (ZFP). All three were detected in under 10 hr. A comparison of the results obtained with those of cytogenetic analysis of the same samples showed a perfect sex match, demonstrating that this PCR technique provides an accurate means of determining gender.