In an ultrasonographic laboratory we scanned 545 consecutive obstetric patients for obstetric indications based on the consultative needs of the physicians involved in their care. We attempted to determine the sex on all fetuses. The gestational age ranged from 15 weeks to term. Fetal sex was established in 196 (36%). After the 25th week of gestation, we predicted the sex in greater than 50%. There were no errors in the entire series. The basis of sex determination was the demonstration of male and female genitalia.