OBJECTIVE To evaluate the role of colour-coded duplex scanning in the preoperative assessment of varicose veins of the lower extremity. METHODS Open study. METHODS District hospital. METHODS 48 patients who were due to be operated on for varicose veins (20 bilateral); 10 were being operated on for the second time. METHODS The planned operation was changed according to the results of the colour-coded duplex scan if they differed from those of physical examination and continuous wave Doppler flow, and its accuracy was verified at operation. RESULTS The use of colour-coded duplex scanning resulted in a change in the plan of operation in 18 of the 68 legs. Escape points between the superficial and deep venous system would have been left intact in 14, the long saphenous vein would have been stripped unnecessarily in three, and one exploration to find an incompetent perforating vein would have been unnecessary. In 6 of the 10 having reoperations colour-coded duplex scanning showed that the recurrences were caused by incompetent perforating veins; this indicated that ligation of perforating veins had been inadequate at the first operation. In the remaining four the duplex scan showed that the recurrence was caused either by a patent incompetent long saphenous vein (n = 2) or by an incompetent saphenofemoral junction (n = 2). CONCLUSIONS Colour-coded duplex scanning is more accurate than physical examination and continuous wave Doppler flow studies in the preoperative assessment of varicose veins of the lower limb.