Strain gauge plethysmography and impedance plethysmography are non-invasive diagnostic techniques for deep venous thrombosis. In 145 consecutive out-patients presented with suspected acute deep venous thrombosis we compared these two techniques. Venography was performed if one or both tests were positive. Both tests were repeatedly normal in 91 (63%) patients, while in 38 (26%) patients both tests were abnormal and venography confirmed the presence of venous thrombosis. In six other patients the impedance plethysmography test result and venography were positive while the result of the strain gauge plethysmography test was normal. This difference was statistically significant (p less than 0.001). Four of these six patients had acute proximal thrombosis of the deep leg veins. The impedance plethysmography was falsely positive in four patients while strain gauge plethysmography was falsely abnormal in one patient. We conclude that in the diagnostic management of patients with suspected deep venous thrombosis, impedance plethysmography is superior to strain gauge plethysmography.