Repeated treatments of localized deep microwave hyperthermia were given to a series of dogs by means of a 915 MHz, water-cooled skirt-type applicator. The applicator was inserted into the rectum and directed toward the prostate in order to heat it by means of the absorbed microwaves while keeping the rectal wall at a lower temperature by surface cooling of the applicator itself. Sessions were given for different lengths of time ranging between ninety minutes and five hours, during which the prostate temperature was kept at 42.5 degrees C (+/- 0.5 degrees C) or 44.5 degrees C (+/- 0.5 degrees C). Three-dimensional temperature distributions in the prostate were measured accurately and verified by a Luxtron Fluoroptic Unit. Temperatures were constantly monitored in the rectal wall and in the prostatic urethra. Thorough and systemic follow-up was done before, during, and after each treatment, and the observations are reported. Two interesting preliminary observations were made: (1) differential blood counts showed significantly monocytosis following the treatments and lasted for at least one week, and (2) values of creatinine phosphokinase (CPK) and serum glutamic oxaloacetic transaminase (SGOT) were found to rise irreversibly in those animals which were later found to have definite histopathologic evidence of localized necrotic damage.