Color duplex flow imaging (CDFI) permits pain- and risk-free direct imaging of the deep venous system of the lower extremities. To prospectively ascertain the accuracy and limitations of this technique, CDFI was performed in 75 lower limbs of 69 consecutive patients referred for venographic evaluation of clinically suspected lower extremity deep venous thrombosis (DVT). The CDFI study was obtained within 24 hours of the contrast venogram. Both studies were interpreted without knowledge of the patient's clinical findings or the results of the other test. Contrast venography was regarded as the standard for diagnosis of DVT. Accuracy was 99% for detection of DVT above the knee and 81% below the knee. Sonographic evaluation of the calf veins was technically adequate in 60% of limbs; accuracy was 98% in this group. In the 40% of limbs with technically limited CDFI studies of the calf, accuracy decreased to 57%. Although small nonocclusive thrombi occurred infrequently in this series of symptomatic patients, CDFI missed three of four such thrombi. It is concluded that CDFI, when not technically compromised, is sufficiently accurate to definitively diagnose symptomatic lower extremity DVT.