Cyclooxygenase 2 (Cox-2) is a key enzyme in prostaglandin synthesis and it has an important role in the pathogenesis of various malignancies. Cox-2 has been studied in solid tumors; however, studies about the role of Cox-2 in non-Hodgkin's lymphomas (NHL) are limited. The aim of this study is to determine the importance of Cox-2 expression in lymphomas. To this end, Cox-2 expression was determined in 177 cases with NHL. In histological terms, 60 cases (33%) had low grade and 117 (67%) had aggressive lymphoma. Ninety-nine cases were found to be positive for Cox-2 (56%); Cox-2 score was between 50 and 100, 101 and 200 and over 200 in 38, 46 and 15 cases, respectively. There was an important association between aggressive histology and Cox-2 expression: Cox-2 was negative in about half of the cases with indolent morphology, while two thirds of the Cox-2 positive cases had aggressive histology (p = 0.036). There was no significant association between Cox-2 expression and clinical-laboratory parameters. Although the overall survival times were longer in cases with lower or no Cox-2 expression as compared with higher Cox-2 expression, the difference was not significant. In conclusion Cox-2 expression is seen about 60% of the cases with NHL and is associated with aggressive morphology.