An electromagnetic blood flow meter was used to study blood flow to the radial forearm flap intra-operatively in 20 patients. Blood flow correlated more significantly with surface area than with weight. Both antegrade and retrograde flow were measured and showed no significant difference. Temperature affected blood flow significantly (P less than 0.01) as did sympathetic block (P less than 0.01). The venous drainage of the flap was studied in five cases. Both superficial and deep systems were equally capable of draining the flap.