Incompetent veins in the popliteal fossa were assessed with duplex ultrasound in 123 legs of 104 patients. In 91 cases the venous reflux was the result of a single incompetent vein draining into the deep system. In 32 cases duplex ultrasound demonstrated a combination of incompetent veins. With the exception of 3 patients duplex scanning accurately localised the incompetent veins and their "junctions" with the deep system. In 48 legs the reflux signal, detected in the surface vein, extended back beyond the "junction" into a proximal segment of the popliteal vein or superficial femoral vein; in most cases the distal popliteal vein remained competent. Six distinct venous reflux patterns were identified in the popliteal fossa, which seemed to correlate with the severity of related clinical symptoms.