Purpose: Multiple studies have indicated that cyclooxygenase-2 (COX-2) inhibitors may prevent colon cancer, which is one of the leading causes of cancer death in the western world. Recent studies, however, showed that their long-term use may be limited due to cardiovascular toxicity. This study aims to investigate whether curcumin potentiates the growth inhibitory effect of celecoxib, a specific COX-2 inhibitor, in human colon cancer cells. Experimental Design: HT-29 and IEC-18-K-ras (expressing high levels of COX-2), Caco-2 (expressing low level of COX-2), and SW-480 (no expression of COX-2) cell lines were exposed to different concentrations of celecoxib (0-50 μmol/L), curcumin (0-20 μmol/L), and their combination. COX-2 activity was assessed by measuring prostaglandin E2 production by enzyme-linked immunoassay. COX-2 mRNA levels were assessed by reverse transcription-PCR. Results: Exposure to curcumin (10-15 μmol/L) and physiologic doses of celecoxib (5 μmol/L) resulted in a synergistic inhibitory effect on cell growth. Growth inhibition was associated with inhibition of proliferation and induction of apoptosis. Curcumin augmented celecoxib inhibition of prostaglandin E2 synthesis. The drugs Synergistically down-regulated COX-2 mRNA expression. Western blot analysis showed that the level of COX-1 was not altered by treatment with celecoxib, curcumin, or their combination. Conclusions: Curcumin potentiates the growth inhibitory effect of celecoxib by shifting the dose-response curve to the left. The synergistic growth inhibitory effect was mediated through a mechanism that probably involves inhibition of the COX-2 pathway and may involve other non-COX-2 pathways. This synergistic effect is clinically important because it can be achieved in the serum of patients receiving standard anti-inflammatory or antineoplastic dosages of celecoxib.