The 20-MHz ultrasonic Doppler probe was used to determine its efficacy as a continuous monitoring technique for microvascular anastomoses. A 1-mm2 piezoelectric crystal embedded in a soft silicone sleeve was sutured directly to the blood vessel distal to the anastomosis. Using the dog femoral artery, simultaneous measurement of velocity and blood flow with an electromagnetic flowmeter established a direct correlation between flow and velocity with a 14 percent error at maximum flow and an 18 percent error at minimum flow conditions. The probe was then implanted in the rabbit femoral artery for 1 week (n = 3) and 4 weeks (N = 6), demonstrating that a continuous tracing could be obtained without injury to the vessel. Our clinical study included 63 patients undergoing free-tissue transfers monitored with the implantable probe for 7 to 29 days (average 10.5 days). Twenty-three flaps were buried. Two patients experienced loss of arterial tracing due to malfunction of the probe (3 percent). Three patients had a venous thrombosis with a present arterial tracing. There were no flap failures per se. All probes were removed without mishap, and there were no complications related to the probe. We conclude that the 20-MHz ultrasonic Doppler probe holds promise as a useful monitoring method.