Medullary thyroid cancer (MTC) is a C cell neoplasm-secreting calcitonin. Surgery remains the only treatment as the primary tumor and metastases resist radio- and chemotherapies. MTC produces high amounts of prostaglandins (PGs). Nonsteroidal antiinflammatory drugs have an antitumoral effect, generally related to the decrease of PG levels. We assessed the therapeutic potential of indomethacin in a model of human (TT cells) tumors in nude mice. Indomethacin (1.5 or 2.0 mg/kg body weight.d for 7 wk) inhibited tumor volume by 49 or 77%, respectively, and decreased the plasma level of CT. Although the terminal deoxynucleotidyltransferase-mediated deoxyuridine triphosphate nick end labeling method revealed few apoptotic nuclei, the number of proliferating cells was significantly decreased (Ki-67 antigen study). Immunological effector recruitment and vascular network was not modified by treatment. The inducible synthesis enzyme, cyclooxygenase-2 (COX-2), was revealed only in infiltrating cells, both in treated and control tumors. The expression of the constitutive synthesis enzyme COX-1 was diminished, and the expression of 15-prostaglandin dehydrogenase, the key enzyme catabolizing PGs, was increased in treated tumors. Thus, our results demonstrated the potential of indomethacin, inhibitor of COX-1 and COX-2, to prevent MTC growth. The synthesis enzyme, COX-1, and the catabolism enzyme 15-prostaglandin dehydrogenase, could be involved in MTC development.