Doppler ultrasound of the deep leg veins. A revolution in the diagnosis of deep vein thrombosis and monitoring of thrombolysis. The diagnosis of lower limb deep vein thrombosis can be made rapidly by compression ultrasound. This noninvasive test has almost completely replaced contrast phlebography in some major hospitals in North America. Compression ultrasound is well suited for making the diagnosis of deep vein thrombosis, measuring its extent, and monitoring the draining veins of the calf for evidence of spread of calf vein thrombosis. The nature of the deep vein thrombus, whether it is obstructing or nonobstructing, can be determined with the additional use of color Doppler mapping, a technique that displays flowing blood as a color overlay to the 2-dimensional real-time ultrasound image. Serial noninvasive determinations of the extent of thrombus in the lower-extremity deep veins is now possible with this combined sonographic imaging approach. Monitoring of possible changes in the extent of the thrombus burden can now be undertaken in patients enrolled in trials assessing the efficacy of different thrombolytic regimens. 