PURPOSE: We previously reported the p27 expression level to be an independent prognostic factor, and a high S-phase kinase-associated protein 2 (Skp2) expression level was significantly correlated with a poor prognosis in patients with gastric cancer. We herein examined the Skp2 expression in breast cancer and attempted to identify any associations between the Skp2 expression status and either the clinicopathologic variables or the patient's prognosis. EXPERIMENTAL DESIGN: We established four Skp2-transfected breast cancer cell lines and assessed the correlations between the Skp2 and p27 expressions using real-time reverse transcription-PCR and a Western blot analysis. We then analyzed the clinicopathologic significance of Skp2 mRNA expression in 169 Japanese patients with breast cancer. An immunohistochemical analysis was also done. RESULTS: The p27 protein expression markedly decreased after Skp2 transfection, whereas no alteration in the p27 mRNA expression was observed. The Skp2 protein expression level as determined by immunohistochemical staining thus showed a significant correlation with the Skp2 mRNA expression (P = 0.001) and a significant inverse correlation with the p27 protein expression (P = 0.042). The patients with a high Skp2 gene expression were significantly younger than those with a low expression (P = 0.002). The prognosis of patients with a high Skp2 expression was significantly (P = 0.022) poorer than for those with a low expression. Moreover, a high expression of Skp2 was an independent variable that correlated with a shorter disease-free survival (relative risk, 3.33; 95% confidence interval, 1.296-8.578; P = 0.013). CONCLUSIONS: The present results suggest that Skp2 may play an important role particularly in young breast cancer, and it is also considered to have strong independent prognostic potential and thus may prove to be a useful target for the treatment of breast cancer.