Transrectal microwave hyperthermia of the prostate was administered to 191 patients with bladder outlet obstruction due to benign prostatic hyperplasia who were either at poor operative risk or who refused surgery. Patients were divided in 2 groups according to age and they underwent either 5 or 10, 60-minute sessions of hyperthermia, with a calculated intraprostatic temperature of 42.5 plus or minus 0.5C. Light and electron microscopy showed no irreversible damage at the glandular epithelium but did demonstrate a significant increase in neoformed intraprostatic capillary-like vessels. At 1, 12 and 24 months residual urine volume was significantly decreased in the majority of patients but only a minor amelioration of urinary flow rates and subjective symptoms was observed. According to maximum flow nomograms all patients were still obstructed postoperatively. Transrectal hyperthermia cannot be considered a genuine alternative to surgery for patients with bladder outlet obstruction due to benign prostatic hyperplasia.