Aldehyde dehydrogenase 1 (ALDH1), a detoxifying enzyme responsible for the oxidation of intracellular aldehydes, was shown to have a function in the early differentiation of stem cells, through its function in oxidizing retinol to retinoic acid. It has been shown that ALDH1 is a predictor of poor clinical outcome in breast cancer. The authors hypothesized that the level of ALDH1 expression may be correlated with the clinical outcome of patients with ovarian cancer. Immunohistochemical staining of ALDH1 expression was analyzed in 442 primary ovarian carcinomas using tissue microarray. The associations between the expression of the ALDH1 and clinical factors (diagnosis, tumor grade, stage, and clinical response to chemotherapy), as well as overall and disease-free survival, were analyzed. Expression of ALDH1 was found in 48.9% of the samples. Fisher's exact test suggested that high expression of ALDH1 was significantly associated with endometrioid adenocarcinoma (P<0.0001), early-stage disease (P=0.006), complete response to chemotherapy (P<0.05), and a low serum level of CA125 (P=0.02). High percentage of cells expressing ALDH1 was associated with a longer overall survival time (P=0.01) and disease-free survival time (P=0.006) by log-rank test. In contrast to its function in breast cancer, ALDH1 was a favorable prognostic factor in ovarian carcinoma. ALDH1 therefore may have a different function in ovarian cancer than it does in breast cancer.