The tumor suppressor gene Smad4 has been localized to chromosome 18q21.1 and is a member of the Smad family that mediates the transforming growth factor β signaling pathway suppressing epithelial cell growth. However, variable expression of Smad4 messenger RNA (mRNA) has been reported, with a loss in some cancers and increased expression in others. The aim of the present study was to investigate the Smad4 mRNA expression in prostate cancer tissues and adjacent noncancerous tissues and its potential relevance to clinicopathological variables and prognosis. The expression change of Smad4 mRNA was detected by using real-time quantitative reverse transcriptase-polymerase chain reaction analysis. The data showed that the Smad4 mRNA expression level in prostate cancer tissues was significantly lower than those in noncancerous tissues. The results indicated that the low expression of Smad4 mRNA in prostate cancer was associated with lymph node metastasis, preoperative prostate-specific antigen (PSA), and Gleason score. Kaplan-Meier survival analysis showed that patients with high Smad4 mRNA expression have longer biochemical recurrence-free survival time compared to patients with low Smad4 mRNA expression. Multivariate analysis revealed that Smad4 mRNA expression was an independent predictor of biochemical recurrence-free survival. Our results emphasize that Smad4 mRNA can be used as a predictive biomarker.