OBJECTIVE To determine the effects on prostate blood flow of heat generated by microwave thermal treatment in patients with benign prostatic hyperplasia. METHODS Prostate blood flow was evaluated by continuous transrectal color Doppler ultrasonography in 2 patients at baseline, after implantation of interstitial needles used for thermal mapping, and during microwave thermal treatment. Temperatures at 30 prostatic, periprostatic, urethral, and rectal sites were continuously monitored. In 1 patient, transrectal prostate compression was applied and the blood flow and temperature response to this maneuver noted. RESULTS Microwave thermal treatment achieved maximum prostate temperatures of 59 degrees C at 5 mm radially from the urethra. Urethral and rectal temperatures remained low. Marked increases occurred in prostate blood flow in response to microwave thermal treatment. These increases were apparent throughout the prostate gland, with the greatest increase in perfusion occurring in the peripheral zone and the posterior half of the transitional zone. After 15 minutes of microwave treatment, peak systolic blood flow increased 99% and 70% in patients 1 and 2, respectively, while end-diastolic blood flow climbed 50% and 112%, respectively. Prostate compression resulted in a prompt quenching of blood flow and an increase in prostate temperature. CONCLUSIONS Based on these preliminary findings in 2 patients, prostate blood flow increases markedly in response to microwave thermal treatment. This compensatory increase in blood flow is likely to be a significant treatment-limiting factor in achieving effective thermoablation.