Amniotic fluid testosterone levels were measured by a sensitive and specific radioimmunoassay technique from 75 term pregnancies and from 57 pregnancies prior to 20 weeks gestation. Statististically significant differences (p < 0.05) were found between levels for term males (123 pg/ml) and term females (63 pg/ml) and between less than 20-week males (176 pg/ml) versus less than 20-week females (60 pg/ml). However, the hormone levels between term males and less than 20-week males were not significantly different from each other, and, similarly, levels between term females and less than 20-week females were not statistically different. In this study, the sex of 37 out of 38 male fetuses and 19 out of 19 females less than 20 weeks gestation could be correctly predicted solely on the basis of amniotic fluid testosterone levels. The implications are apparent for genetic counseling by using this rapid methodology for intrauterine sex determination instead of, or in addition to, full chromosome analysis.