Thermography, a fast, simple, and sensitive method, was compared with conventional contrast medium phlebography in the diagnosis of acute deep venous thrombosis of the lower extremities in 141 patients. In 84% of the cases, the results with both techniques agreed. Thermograms showed the pathologic findings in 38 of 41 cases with deep thrombosis. In the three cases with false-negative findings, only the anterior surfaces of the legs were studied. In 19 cases, pathologic thermograms were recorded in the absence of deep thrombosis; the main reasons for findings were posttraumatic, postoperative, and postthrombotic states, and insufficiency of the perforant veins. If a patient has no previous history of a venous or other intervening disease, thermography with a negative finding excludes deep venous thrombosis. If it is used as a primary method, it can replace many phlebographies in the diagnosis of acute deep venous thrombosis.