Clinical evaluation of deep venous thrombosis is often unreliable. Recently it has been shown that deep venous thrombosis may be detected with colour duplex scanning which is safer than phlebography. However it is not clear how colour duplex correlates with phlebography. One hundred and twenty consecutive patients with suspected deep vein thrombosis were studied by colour duplex scanning and phlebography to compare the two methods. Both tests were positive in 102 limbs and negative in 16 limbs. In two cases of below-knee thrombosis colour duplex was negative while phlebography showed small localised thrombosis. The sensitivity of colour duplex was 98%, its specificity and positive predictive value 100% and its negative predictive value 88.8%. In conclusion colour duplex appears to be as effective as phlebography in detecting deep venous thrombosis above knee.