Color flow imaging combined with compression ultrasonography were used to prospectively evaluate symptomatic patients first seen with suspected deep vein thrombosis during pregnancy. The incidence of symptomatic deep vein thrombosis (7 of 10,000 deliveries) shown prospectively by ultrasonography was similar to that of historic controls who had a diagnosis confirmed by venography (11 of 26, 191 deliveries). A preferential pattern of proximal left-sided iliac or femoral vein involvement was common to both cohorts, being seen in five of the seven cases diagnosed by ultrasonography and eight of the eleven cases diagnosed by venography. We conclude that ultrasonography, consisting of compression ultrasonography and color flow imaging, can replace venography in the diagnosis and evaluation of the extent of symptomatic deep vein thrombosis during pregnancy.