p27KIP1 is a member of the CIP/KIP family of cyclin-dependent kinase inhibitory proteins that negatively regulate cell proliferation. Recent studies reported decreased p27 expression in breast and colon carcinomas and found that the loss of p27 is associated with a poor prognosis. We report here the results of our immunohistochemical analysis of p27 in human prostate cancer. Formalin-fixed, paraffin-embedded, whole-mount sections of prostate cancer from 73 selected patients treated by radical retropubic prostatectomy were obtained from the Department of Pathology, The Methodist Hospital, Houston, Texas. Ten histologically normal and nine high-grade prostatic intraepithelia neoplasia foci were selected from these whole-mount sections, and nine cases of transplant donor prostates were chosen as controls. Also, 10 prostate cancer metastatic lymph nodes were used to compare with the primary cancer group. Sections were immunostained with a monoclonal antibody against p27 protein using the avidin-biotin complex immunohistochemical method. Immunoactivity was evaluated without knowledge of follow-up and recorded as the p27 labeling index (LI) (defined as the percentage of p27-positive cells among epithelia of the same category). The p27 (LI) in normal prostatic epithelia was 86.4+/-3.5% (the mean +/- the standard error of the mean). In contrast, the p27 immunoreactivity was significantly lower in cancers (LI: 43.5 +/-3.7%, P < .001) and in the high-grade prostatic intraepithelial neoplasia group (LI: 59.3 +/- 3.2%, P < .05). Expression of p27 in the metastatic lymph node group was significantly lower than in the other groups, including the prostate cancer cases and the cases of high-grade intraepithelial neoplasia (LI, 7.0%; P = .05). There was no association of the mean p27 LI with progression after radical prostatectomy. Nonrecurrent cases, with a mean follow-up time of greater than 5 years (n = 45), equalled 41.9%; recurrent cases, with a mean follow-up time of 18.3 months (n = 28), equalled 40.0%. The mean p27 LI was not associated with pathologic stage. Organ-confined specimens (n = 21) equalled 34.2%; cases of extraprostatic extension (n = 24) equalled 46.5%; and samples showing seminal vesicle involvement (n = 14) equalled 47.6%. In 14 cases with lymph node metastases, the mean p27 LI was 48.1% in the primary cancer (P = .2322). There was no association of the mean p27 LI with the Gleason score (P = .4747) nor with the clinical stage (P = .9914).