The free radial forearm flap has two drainage veins, the cutaneous and the deep, but no established consensus has been reached on the selection of the drainage pedicle. In our study, the flow volumes of the veins were examined by colour Doppler ultrasonography after 20 forearm flaps had been raised. The volume through the deep vein was significantly higher than that through the cutaneous vein (p < 0.01). In comparing the total flow (both veins open), the flow rate through the deep vein alone (cutaneous vein occluded) was over 80% in 13 cases, 60%-80% in seven, and under 60% in none; that through the cutaneous vein alone (deep vein occluded) was 60%-80% in eight, 40%-60% in eight, under 40% in four, and over 80% in none. Our results show the importance of the deep vein, as indicated by its high drainage capacity from the early stages of flap transfer.