This prospective study evaluated the tissutal, cellular and intracellular effects of transrectal microwave hyperthermia on human benign prostatic hyperplasia. Forty-eight patients with benign prostatic hyperplasia underwent ten 60-min-long sessions of transrectal hyperthermia with an intraprostatic calculated temperature of 42 +/- 0.5 degrees C. Ultrasound-guided transperineal biopsies of the prostate were taken before and 1 month after completion of treatment. Postoperatively, morphometric analysis of bioptic specimens showed a statistically significant (p < 0.01) increase in the number of intraprostatic arterioles and capillary-like vessels. Diffused inflammatory infiltrates were also noted. Postoperative integrity of intracellular organelles and cellular membranes was evidenced by transmission electron microscopy. Our regimen of transrectal prostatic hyperthermia did not cause any irreversible histological or ultrastructural damage to the prostatic tissue. Hyperthermia-induced increase in blood flow could enhance drug delivery to the prostate gland.