p27Kip1 is a cyclin-dependent kinase inhibitor that negatively regulates cell proliferation by mediating cell cycle arrest in G1. This study was undertaken to assess the prognostic value of p27Kip1 in localized human prostate cancer. Archival material from 113 radical prostatectomy specimens obtained between 1985 and 1993 was stained immunohistochemically for p27Kip1 protein using a commercially available antibody. Patient charts were reviewed for preoperative serum prostate-specific antigen, clinical and pathological staging, Gleason tumor grade, time to biochemical and clinical recurrence, and survival. Strong p27Kip1 staining was uniformly seen in benign prostatic epithelial components in all tumor sections. p27Kip1 staining was reduced in most prostate cancers and was variable in prostatic intraepithelial neoplasia. Decreased p27Kip1 staining (<25% of nuclei stained positive for p27Kip1) correlated with seminal vesicle involvement (P = 0.0032) and with higher Gleason grade (P = 0.0114). On univariate analysis, low p27Kip1 predicted an increased risk of treatment failure in the node-negative cohort (P = 0.0037) and in the subset who did not receive neoadjuvant hormonal therapy (P = 0.049). Low p27Kip1 expression was an independent predictor of treatment failure on multivariate analysis of lymph node negative prostate cancers following radical retropubic prostatectomy (n = 102; P = 0.047). Seminal vesicle involvement (P = 0.034) and positive surgical margins (P = 0.047) were also independent prognostic factors for disease recurrence. In patients who received preoperative neoadjuvant hormonal therapy, low p27Kip1 in the pathological specimen was an even stronger predictor of outcome than it was in the entire group (n = 23, P = 0.015).