In a prospective study, a small team of obstetricians concurrently performed midtrimester amniocentesis for prenatal genetic diagnosis on 32 patients after ultrasonic placental localization and on 50 patients without prior ultrasound. The use of ultrasound did not affect the red cell count in the fluid samples. Furthermore, the 2 patient groups had similar rates of grossly bloody taps. The average number of viable clones after culture was lower both after bloody taps and when ultrasound had been used, but in neither case was the difference significant. There were no postamniocentesis complications in the entire patient population. These results suggest that ultrasonic placental localization is not helpful in avoiding bloody taps, and that bloody taps may not necessarily be dangerous. Therefore, since the long-range hazards of in utero exposure to ultrasound may not yet be known, it would seem judicious to use preamniocentesis ultrasound selectively and on specific indication, rather than routinely.