This study was designed to investigate the expression of cyclooxygenase (COX)-2 in ovarian serous tumors (benign, borderline tumors, and carcinomas) and primary peritoneal serous carcinomas. Cases diagnosed between 1995 and 2001 were reviewed; 47 benign tumors, 6 borderline tumors, and 39 carcinomas were examined, as well as 12 normal ovaries that served as controls. Blocks were stained with anti COX-2 polyclonal antibody and staining was graded qualitatively. The staining intensity was assessed as weak (score of 1), moderate (score of 2), or strong (score of 3). Normal ovarian and tubal epithelium, inclusion cysts, benign serous tumors, and borderline tumors had a uniform score 3 staining pattern. Serous ovarian carcinomas had variable staining scores, tending to correlate with the level of tumor differentiation. Well-differentiated carcinomas had more intense COX-2 staining than poorly differentiated carcinomas, which had only weak COX-2 staining. The degree of COX-2 staining was not significantly related to overall survival. In conclusion, COX-2 expression is present in serous tumors, including benign tumors, borderline tumors, and carcinomas. Similar to the findings in other neoplasms, COX-2 expression is strongest in well-differentiated tumors and is much less evident in those that are poorly differentiated. The clinical utility of these findings is related to the potential role of nonsteroidal anti-inflammatory drugs, which are COX-2 inhibitors, in treating and/or preventing some forms of ovarian carcinoma.