By using an ultrasonic Doppler blood velocity detector and manual compression of tissues at various levels to determine abnormalities of flow in femoral, popliteal, and posterior tibial veins, accuracy of the method was determined in 248 extremities by performing concurrent phlebography. The Doppler examination agreed with phlebography in 75.9% (63 of 83) of the limbs containing venous thrombosis and 90.9% (150 of 165) of the normal limbs. The method cannot detect clot when only tributary veins are involved and when patent collateral veins are present. Error may also arise from improper positioning of the probe, or emptying the veins by gravity or compression bandage. The method is useful as a screening procedure when used in conjunction with the clinical examination, particularly in thromboses involving the femoral vein and in those that appear to be recent.