Prostate cancer (PCa) is the most common male cancer in the United States. A major challenge that remains is to predict the clinical outcome in managing PCa patients. The prolyl isomerase Pin1 has been shown to be overexpressed in some human cancer tissues and thought to be an important player in several oncogenic pathways. However, the relationship between Pin1 expression and clinical outcome of cancer patients has not been explored. In this study, we investigated the role of Pin1 in human PCa progression and its clinicopathological significance. Immunohistochemical assessment with affinity-purified polyclonal Pin1-specific antibodies was performed on formalin-fixed paraffin sections of tissue microarray composed of 580 radical prostatectomy specimens. As determined by visual semiquantitation and confirmed by automated image analysis quantitation, Pin1 expression was positively correlated with clinical stage. Furthermore, Cox survival analysis results indicated that patients with a higher Pin1 expression had a significantly higher probability of recurrence than their counterparts with low Pin1 expression, as defined by a serum prostate-specific antigen level of > or =0.4 ng/ml on two consecutive occasions after radical prostatectomy. In addition, patients with high Pin1 expression had almost 4 times the risk of having earlier recurrence than those with low Pin1 expression; patients with a very high level had 8.1 times the risk of an earlier recurrence than a low Pin1 expresser. Pin1 was also an excellent predictor of recurrence in the subset of patients with Gleason score 6 or 7 when analyzed separately: a patient with high Pin1 expression had 8.6 times the risk of having earlier recurrence than one with low Pin1 expression. Pin1 expression is as good as or better than currently used postoperatively available clinicopathological parameters and potentially could be used in the preoperative setting to assist in choice of treatment. Thus, this study suggests a role for Pin1 expression as a potentially excellent prognostic marker in PCa and suggests that Pin1 may also serve as a novel therapeutic target for PCa.