We reviewed 157 patients retrospectively with incidental carcinoma of the prostate who had been treated at our collaborating hospitals during the past ten years. Of 5212 patients with benign prostatic hyperplasia who received subcapsular prostatectomy or transurethral resection of the prostate (TUR-P), 157 (3.0%) were diagnosed as having an incidental carcinoma of the prostate, which was somewhat lower than that in previously published reports. Of these, 30 and 127 patients were in stage A1 and A2, respectively. Well, moderately and poorly differentiated carcinomas were found in 44.6%, 36.7% and 18.5% of the patients, respectively. The incidence of poorly differentiated carcinoma in the study seemed to be higher than that in the previous reports. A positive correlation was identified in TUR-P specimens between the carcinoma differentiation and its extension which was evaluated by cancer-positive chip ratio. Atypical adenomatous hyperplasia and intraductal dysplasia were identified in 36.9% and 85.3% of the patients with incidental carcinoma, respectively. These incidences tended to become lower as the carcinoma became less differentiated or more extended. Further studies will be necessary to define the significance of these pathological findings as a direct biological precursor of prostatic carcinoma. Six out of the 157 patients with incidental carcinoma showed a progression during the follow-up period. All of these patients were in stage A2 and all but one showed a histology of moderately or poorly differentiated carcinoma at the time of diagnosis. Radical prostatectomy or radiation therapy as well as endocrine therapy should be considered as treatment modalities for stage A2 patients, when staging lymphadenectomy shows no pelvic lymph node metastasis.