Colour Doppler ultrasound is a new technical development allowing simultaneous grey scale imaging and a dynamic colour flow vascular image. To date, many real time ultrasonic studies have been assessed in the diagnosis of lower limb venous thrombosis and have been shown to be accurate in the femoral and popliteal segments. A double blind prospective study comparing colour Doppler with contrast venography in the diagnosis of lower limb thrombosis was performed in a group of 40 patients. Of the study group, 26 venograms were negative and 27 ultrasound examinations were negative. Of the 14 positive venograms there was one false negative colour doppler scan which missed a calf and lower popliteal thrombosis. Two cases of isolated calf vein thrombosis were successfully detected by colour Doppler. Overall, the sensitivity and specificity for detection of lower limb venous thrombosis, including calf vein assessment, were 93% and 100% respectively. Colour Doppler is easy to perform; the average scanning time being 15 minutes for complete assessment of a unilateral lower limb venous system. Spontaneous flow is evident in the femoropopliteal segment, whilst proximal calf vein flow can only be appreciated with the aid of distal compression. Eccentric thrombus and partially recanalized thrombus can be shown. Although the number of isolated calf vein thromboses was small, early experience suggests colour Doppler may be useful in the assessment of proximal calf vein patency.