Recent investigations have shown that hyperthermia can reduce the volume of the prostate and improve local symptoms in patients with carcinoma of the prostate. Histological examinations of the effect of hyperthermia on prostatic cancer tissue, however, have not been performed systematically until now. Thus, we initiated a study to investigate the effects of heat on prostatic cancer as a prerequisite for further clinical trials on hyperthermia as treatment for prostatic cancer. Twenty patients with untreated prostatic cancer underwent local hyperthermia (915 MHz), each receiving four sessions of 60 min each. The intraprostatic temperature was 42-43 degrees C. Histological specimens of the prostate were taken before the treatment and 1-2 weeks after the last hyperthermia session. Hyperthermia produced hyperaemic alterations of the prostatic stroma and a diffuse oedema with interstitial lymphoplasmacellular infiltration. Definite signs of tumour cell necrosis, however, could not be seen in any of the patients. Hence the shrinkage of prostatic tumours described earlier cannot be explained by histologically proven tumour cell destruction. Thus hyperthermia is not adequate as a single treatment for prostatic cancer. Hyperthermia may, however, be useful as part of integrated therapy regimens together with cytostatic or hormonal agents and radiotherapy because of hyperaemic, chemo- and radiosensitizing effects.