Objective: This study was undertaken to investigate the ability of focused ultrasonic surgery to occlude blood flow in vivo. Study Design: A 5-mm linear track exposure of 1.7-MHz focused ultrasound was applied across the femoral vessels for 5 seconds. Free field spatial peak intensities in the range of 1000 to 4660 W · cm^–2 were used. Vascular occlusion was confirmed after demonstration of an absent distal arterial pulse and an absent flow signal on magnetic resonance angiography and subtracted (after minus before) contrast-enhanced dual-echo steady-state sequences. Results: The minimum intensity for consistent vascular occlusion was 1690 W · cm^–2 at a focal depth of 5 mm when the transducer was moved at 1 mm · s^–1 orthogonal to the direction of blood flow. Conclusions: This study demonstrates that focused ultrasonic surgery can achieve reproducible vascular occlusion in vivo. Potential obstetric applications include noninvasive ultrasonographically guided occlusion of placental vessels mediating interfetal transfusion in monochorionic twins. (Am J Obstet Gynecol 2000; 182:387-92.)