A large series of repeated experiments were performed applying localized microwave hyperthermia to the prostate in dogs using a new water-cooled skirt-type antenna [1], operating at 915 MHz, as part of a new hyperthermia apparatus being developed for the treatment of the prostate in humans. The prostate gland of 20 male dogs was heated repeatedly under general anesthesia, at temperatures between 40 degrees C and 47 degrees C, and for different lengths of time up to 10 h. The prostate and other tissues were evaluated histopathologically following treatments. Invariably, all treatments by hyperthermia of the prostate caused a mononuclear inflammatory infiltration in the interstitium and polymorphonuclear infiltration in the glandular elements. Permanent tissue damage was found to be time-and temperature-dependent. Heating at 42.5 degrees C (+/- 0.5 degrees 5C) for up to 1.5 h was found to be harmless and could be safely repeated with our equipment. This study was part of a preclinical evaluation of a new antenna and apparatus.