In 856 patients over a 6-year period, amniocentesis was performed in the midtrimester for prenatal genetic counseling. The use of ultrasound scanning of the gravid uterus prior to the procedure reduced the incidence of bloody taps threefold and the withdrawal of grossly bloody amniotic fluid by fourfold. The best results (7.6% overall and 2.0% grossly bloody taps) were obtained when the procedure was performed by an operator not only experienced and adept at intrauterine manipulation but who had training and experience in ultrasonography as well. These results contrast favorably with that of operators utilizing but not trained in ultrasound scanning (36% overall, 8% grossly bloody taps) and of those who had the uterus "mapped out" by a sonographist but performed the amniocentesis later in their offices (18% overall, 12.1% grossly bloody taps). Other advantages of prior sonography are also discussed.