p27^Kip1 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 p27^Kip1 in localized human prostate cancer. Archival material from 113 radical prostatectomy specimens obtained between 1985 and 1993 was stained immunohistochemically for p27^Kip1 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 p27^Kip1 staining was uniformly seen in benign prostatic epithelial components in all tumor sections. p27^Kip1 staining was reduced in most prostate cancers and was variable in prostatic intraepithelial neoplasia. Decreased p27^Kip1 staining (<25% of nuclei stained positive for p27^Kip1) correlated with seminal vesicle involvement (P = 0.0032) and with higher Gleason grade (P = 0.0114). On univariate analysis, low p27^Kip1 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 p27^Kip1 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 p27^Kip1 in the pathological specimen was an even stronger predictor of outcome than it was in the entire group (n = 23, P = 0.015).