The tumor suppressor gene Smad4 (DPC4) has been localized to chromosome 18q21.1 and is a member of the Smad family that mediates the transforming growth factor beta signaling pathway suppressing epithelial cell growth. However, variable expression of this protein has been reported, with a loss in some cancers and increased expression in others. Given both the variability and lack of consensus reported regarding Smad4 expression in prostate cancer, we assessed Smad4 immunoreactivity in prostatic adenocarcinomas (PACs). Formalin-fixed, paraffin-embedded tissue sections from 133 PACs were immunostained by a manual method using indirect biotin streptavidin horseradish peroxidase and diaminobenzidine detection using a monoclonal mouse antihuman Smad4 antibody (sc-7966; Santa Cruz Biotechnology Inc, Santa Cruz, Calif). Nuclear immunoreactivity and cytoplasmic immunoreactivity were each semiquantitatively scored based on intensity and percentage of positive cells. Deoxyribonucelic acid ploidy was determined on Feulgen-stained tissue sections by static image analysis. Results were correlated with morphological and prognostic variables. Variable nuclear and cytoplasmic Smad4 positivity was noted in the adjacent benign glands in all cases. Of 133 PACs, 64 (48%) featured increased nuclear and 68 (51%) featured increased cytoplasmic protein expression. Nuclear Smad4 overexpression correlated with tumor grade (P = .02), stage (P = .04), and DNA ploidy (P = .04). Cytoplasmic overexpression correlated with tumor grade (P = .04) and DNA ploidy (P = .04) while showing a trend for correlation with tumor stage (P = .08). Neither nuclear nor cytoplasmic Smad4 overexpression correlated with postsurgical biochemical disease recurrence. Smad4 protein expression persists in PACs compared with benign glands, with both nuclear and cytoplasmic overexpression correlating with prognostic variables indicative of aggressive tumor behavior. Given the significant reported variability of Smad4 in several different cancers, further studies in prostate and other tumors are warranted to elucidate its role in tumorigenesis.