Real-time ultrasonography was performed on both legs of 104 patients with suspected deep-vein thrombosis of the legs. The results were compared with those of phlebography (146 one-extremity phlebograms) independently performed. During the first part of the study (69 individual phlebograms) only the upper leg and popliteal region were assessed: absent compressibility of the vessel lumen was the only criterion of thrombosis. In the second part (77 phlebograms) the lower leg was also investigated: non-compressible, echo-poor vessels were typical of thrombosis. Phlebography demonstrated 21 thromboses of the common femoral vein, 36 of the superficial femoral vein, and 41 of the popliteal vein. The 77 phlebograms in the second part demonstrated 29 thromboses of the deep veins of the lower leg. Sonography had a sensitivity of 100% and a specificity of 99% for the region of the common femoral vein, 95% and 99%, respectively, for the superficial femoral vein, 95% and 99% for the popliteal vein and 89% and 100% for the deep veins of the lower leg. Real-time ultrasonography of the deep-vein system of the leg is thus of great diagnostic value both for the upper and the lower leg and is very useful as the initial physical method of examination if thrombosis is suspected.