The newly identified 3p21.3 tumor suppressor gene RASSF1A is inactivated by hypermethylation in variable solid tumors, including those of the lung, breast, prostate, kidney, and ovary. The purpose of this study was to evaluate the methylation status of RASSF1A in various types and stages of ovarian epithelial tumors. We analyzed the DNA methylation status of ovarian tumors using methylation-specific polymerase chain reaction in 54 frozen ovarian tumor tissues and in 97 cases of archival ovarian serous epithelial tumors using a microdissection procedure. Hypermethylation statuses were examined vs clinicopathologic findings. RASSF1A promoter methylation rates in the various types of fresh ovarian tissues were as follows: serous cystadenoma (1/5), serous tumor of borderline malignancy (2/7), serous adenocarcinoma (4/10), mucinous cystadenoma (0/5), mucinous tumor of borderline malignancy (2/7), mucinous adenocarcinoma (3/6), transitional-cell carcinoma (1/3), clear-cell carcinoma (3/3), and malignant müllerian mixed tumor (3/3). In archived serous tumor tissues, RASSF1A promoter hypermethylation was detected in serous cystadenoma (1/6, 16.6%), serous tumor of borderline malignancy (20/41, 48.8%), and in serous adenocarcinoma (25/50, 50%). The status of RASSF1A hypermethylation in borderline tumors was found to correlate statistically with the presence of microinvasion (p=0.002), peritoneal implant (p<0.001), and bilaterality (p=0.019). The RASSF1A promoter hypermethylation was frequently found in borderline tumors and carcinomas, suggesting that RASSF1A promoter hypermethylation may be a useful molecular marker for the early detection of ovarian tumors.