In an unsclcctcd, consecutive series of 137 midtrimester amniocenteses for prenatal genetic diagnosis, it was found that the incidence of blood contamination of amniotic fluid samples was similar regardless of placenta! site as determined by ultrasound localization. Three (5.8%) patients with anterior placental location versus 5 (9.1%) patients with posterior placentas had gross blood in the amniotic fluid by visual inspection. These results support the view that placenta) penetration during amniocentesis is not the major cause of bloody taps. Ultrasound studies done prior to midtrimester amniocentesis should, perhaps, be more specifically evaluated in relation to pregnancy information obtained rather than solely as an aid to improve technical performance of amniocentesis.