As a putative marker for cancer stem cells in human malignant tumors, including ovarian cancer, CD133 expression may define a tumor-initiating subpopulation of cells and is associated with the clinical outcome of patients. However, at this time its clinical significance in ovarian cancer remains uncertain. The aim of this study was to clarify the clinical role of CD133 expression in human ovarian cancer. Immunohistochemical staining of CD133 expression was performed in 400 ovarian carcinoma samples using tissue microarray. The associations among CD133 expression and clinical factors (diagnosis, tumor grade, cancer stage, and clinical response to chemotherapy), overall survival and disease-free survival time were analyzed. CD133 expression was found in 31% of ovarian carcinoma samples. Fisher's exact test and one-way analysis of variance suggested that CD133 expression was associated with high-grade serous carcinoma (P=0.035), late-stage disease (P<0.001), ascites level (P=0.010), and non-response to chemotherapy (P=0.023). CD133 expression was also associated with shorter overall survival time (P=0.007) and shorter disease-free survival time (P<0.001) by log-rank test. Moreover, CD133 expression was an independent predictor of shorter disease-free survival time in an unconditional logistic regression analysis with multiple covariates (P=0.024). Our results thus show that CD133 expression is a predictor of poor clinical outcome for patients with ovarian cancer, supporting the proposed link between CD133 and cancer stem cells.