A new technique is presented for monitoring microvascular free tissue transfer. When performing free myocutaneous latissimus dorsi flap transfer to the lower limb, the flap artery and vein often lie on top of each other. Using conventional CW or pulsed ultrasound Doppler flowmeters, it is impossible to examine the patency of arterial and venous anastomoses simultaneously. However, a bidirectional CW ultrasound Doppler system and its spectral analyzing display can simultaneously examine both anastomoses, by separating arterial inflow from venous backflow multifrequencies.