To understand the potential role of cyclooxygenase (COX) in normal and inflammatory human diseases, we characterized the expression of COX-1 and COX-2 in biopsies of osteoarthritis, atherosclerosis, and cancer. Tissues were prepared for immunohistochemistry by standard methods, and representative cases assayed via Western blot and quantitative RT-PCR. COX-2 was not detected in normal human tissues with few exceptions. Moderate to marked COX-2 was observed in the macula densa (MD) and thick ascending limb (TAL) in human fetal kidneys, but was not detected in neonatal and adult MD and TALs. Low level, constitutive COX-2 was detected in colonic epithelium, peribronchial glands, and pancreatic ductal epithelium. Low to moderate COX-2 was detected basally in the cortex, hippocampus, hypothalamus, and spinal cord, and in reproductive tissues during ovulation, implantation and labor. No COX-2 was detected in the existing vasculature in normal tissues, and was also not expressed throughout the ductus arteriosus. COX-2 was markedly induced in human tissues of osteoarthritis, atherosclerosis and cancer. COX-2 was prominently expressed in the synovium, fibrocartilage of osteophytes, and in the blood vessels in the osteoarthritic (OA) knee joint. COX-2 was also prominently detected in the macrophages/foam cells in atherosclerotic plaques, and in the endothelium overlying and immediately adjacent to the fibrofatty lesion. Moderate- to intense COX-2 expression was consistently observed in the inflammatory cells, neoplastic lesions, and blood vessels in all epithelial-derived human cancers studied. In contrast, COX-1 was relatively ubiquitously observed in both normal and pathophysiological conditions. These data collectively imply COX-2 plays an important role in mediating a variety of inflammatory diseases, and imply COX-2 inhibitors may be effective in the prevention and/or treatment of OA, heart disease, and epithelial cancers.