Background and Objectives: p27Kip1 is an inhibitor of cyclindependent kinases and is speculated to be a potential prognostic indicator in numerous human cancers. We investigated expression of p27Kip1 along with cyclin D1 in gastric cancer to estimate the clinical utility of p27Kip1. Methods: Immunohistochemical assay for p27Kip1 and cyclin D1 proteins was performed in 64 patients with primary gastric cancer. Correlation between p27Kip1 expression and clinical-biological parameters including patient survival was analyzed. Results: p27Kip1 expression was suppressed in 40 (62.5%) of 64 gastric cancer patients and cyclin D1 was overexpressed in 22 (34.4%) out of 64. Expression of p27Kip1 was significantly reduced in poorly differentiated cancers (82.1%, 23/28; P ס 0.015) and was also reduced in the tumors with high S-phase fraction (86.7%, 26/30) compared with tumors showing low S-phase fraction (41.2%, 14/34; P ס 0.0002). Expression of p27Kip1 and cyclin D1 was inversely correlated (P ס 0.021). In univariate analysis, extent of the disease (P < 0.001), expression of cyclin D1 (P ס 0.0001), and reduced expression of p27Kip1 (P ס 0.0006), were statistically significant to predict patient's outcome, but depth of invasion (P ס 0.008) and pathologic stage (P ס 0.009) emerged as significant prognostic indicators in multivariate analysis. Conclusion: Expression of p27Kip1 is closely linked with cell proliferation and differentiation of human gastric cancer. p27Kip1 seems to have potential as a prognostic marker in the management of gastric cancer patients.