Objective: To investigate the role of cyclin G1 and cyclin B1 in ovarian tumorigenesis. Study Design: We examined cyclin B1 and G1 expression in 58 epithelial ovarian cancer, 18 low-malignant-potential ovarian tumors, and 6 normal ovarian epithelium samples using immunohistochemistry. We also examined cyclin G1 and p53 expression in 7 epithelial ovarian cancer cell lines using Western blot analysis. Results: Nuclear cyclin B1 expression was significantly higher in low-malignant-potential tumors than in normal ovarian epithelium. There was no difference in nuclear or cytoplasmic cyclin B1 or cyclin G1 expression between epithelial ovarian cancer and normal ovarian epithelium. Cyclin G1 and B1 expression was not associated with p53 expression or clinicopathologic features in patients with epithelial ovarian cancer or low-malignant-potential tumors. Conclusion: Our data demonstrated that nuclear cyclin B1 is overexpressed in low-malignant-potential tumors, which may contribute to the development of low-malignant-potential tumors. Cyclin B1 and G1 may not be suitable targets for epithelial ovarian cancer treatment. © 2009 Mosby, Inc. All rights reserved.