Cyclooxygenase-2 (COX-2), the enzyme that converts arachidonic acid to prostaglandin H2, is expressed in normal brain and kidney, activated macrophages, synoviocytes during inflammation, and malignant epithelial cells. COX-2 expression is stimulated by a number of inflammatory cytokines, growth factors, oncogenes, lipopolysaccharides, and tumor promoters. There is evidence that COX-2 plays a key role in tumorigenesis through stimulating epithelial cell proliferation, inhibiting apoptosis, stimulating angiogenesis, enhancing cell invasiveness, mediating immune suppression, and by increasing the production of mutagens. Results of several studies using mouse models of colon cancer and the results of clinical trials have shown COX-2 to be a useful target for the prevention and treatment of colon cancer. Studies with several other epithelial cancers involving different organ sites, e.g., breast, prostate, bladder, lung, and pancreas, suggest that COX-2 plays an important role in the pathogenesis of these cancers. In this review, we summarize the studies that pertain to the involvement of COX-2 in breast cancer. COX-2 overexpression affects the physiological processes at different organ sites in a similar manner, although specific effectors and targets of COX-2 may differ at different sites. Thus in reviewing the data on the involvement of COX-2 in breast cancer, we have also considered the findings regarding the role of COX-2 in other organ sites. Studies from mouse models of mammary tumorigenesis and from human breast cancer cell lines provide evidence that COX-2 overexpression plays an important role in the pathogenesis of malignant breast cancer in humans. Because of availability of effective and relatively safe COX-2 inhibitors, it should be soon possible to evaluate their effectiveness in the clinic for the prevention and treatment of breast cancer. It is likely that the COX-2 inhibitors will be effective in the treatment regimens involving combination chemotherapies.