This article has briefly reviewed the diagnosis of DVT by clinical evaluation and by impedance plethysmography. Clinical evaluation alone cannot be relied on for patient management, but when carefully performed, it remains useful in determining the need for additional testing. The medical history, especially the detection of risk factors for DVT, is as important as the physical examination. In 1,464 patients suspected of DVT, the incidence of this disease by impedance plethysmography was only 11 percent when there were no major risk factors present but increased to 50 percent when three risk factors were present. Impedance plethysmography is a widely used noninvasive test for the diagnosis of DVT. It has a sensitivity and specificity of 94 percent based on correlation with 2,561 venograms. The method is based on quantitation of the venous outflow from the calf after temporary venous occlusion in the thigh. It is particularly helpful in ruling out DVT in patients with suspicious signs or symptoms. Recent improvements in methods, including computer-assisted data analysis, appear likely to further extend its usefulness.