Clinical and immunohistochemical studies were conducted to evaluate prostatic papillary adenocarcinoma and prostatic papillary hyperplasia. Subjects consisted of 5 cases of papillary adenocarcinoma and 2 cases of papillary hyperplasia. There is no conclusive clinical factor for preoperative diagnosis, but we attach importance to endoscopic findings. PSA, PAP, high molecular weight cytokeratin, and PCNA were evaluated immunohistochemically. PSA became positive in every instance but one--a case of papillary adenocarcinoma which became +/-. PAP was + in all cases, except for 1 case of papillary adenocarcinoma. Basal cells were positive for high molecular weight cytokeratin in 2 cases of papillary hyperplasia but were missing in papillary adenocarcinoma. Although PCNA was free from positive nuclei in papillary hyperplasia, positive nuclei were found in all cases of papillary adenocarcinoma. Considering these immunohistochemical results, papillary adenocarcinoma can be said to originate in the glandular epithelium of the prostate, as does ordinary prostatic carcinoma.