p27kip1 (p27) protein is an inhibitor of cyclin and cyclin-dependent kinase complexes and prevents progression of cells from G1 to the S phase of the cell cycle. p27 might have tumor suppressor activity, and decreased p27 expression is associated with aggressive tumor behavior in several human malignancies. The object of this study was to evaluate p27 expression in prostatic adenocarcinoma treated by radical prostatectomy and to assess its association with numerous morphologic and clinical features. One hundred thirty-eight prostatic adenocarcinomas were evaluated for p27 expression by quantifying nuclear immunohistochemical staining. p27 expression was tested for association with patient age, family history of prostate cancer, preoperative serum prostate-specific antigen level, Gleason score, extraprostatic extension, seminal vesicle involvement, lymph node metastases, tumor-node-metastasis stage, DNA ploidy by flow cytometric analysis, and subclinical biochemical failure. p27 expression was analyzed as a continuous variable, and we also classified the tumors as low expressors (< 50% of cells p27 positive) or high expressors (> 50% of cells p27 positive) for comparison. Patients with adenocarcinomas that exhibited low p27 expression had higher mean Gleason scores than did high expressors (7 vs. 6.2, respectively; P = .002). Low p27 expression correlated with positive surgical margins (P = .05), seminal vesicle involvement (P = .007), lymph node metastasis (P = .03), and aneuploid cancers (P = .003), but it did not correlate with subclinical biochemical failure. p27 expression correlated with a number of prognostic morphologic features in prostatic adenocarcinoma, and the evaluation of p27 expression might provide additional prognostic information.