Duplex spectral or color Doppler sonography demonstrated major abdominal vessel blocks affecting 18 vessels in 16 infants aged between 23 and 56 weeks postconceptional age. The portal vein was affected in 8 infants, the renal vein in 3, the renal vein and inferior vena cava in 1, the renal artery in 1, the inferior vena cava and hepatic vein in 1, and the aorta in 2 infants. Doppler sonography confirmed the grey-scale findings and often demonstrated the extent of vascular flow, providing functional information not available from the grey-scale scan. It facilitated the follow-up of thrombosis during therapy. Most important of all, in many instances it enabled a rapid and reliable specific diagnosis often not possible on the grey-scale study alone and hence altered patient management.