Treatment with the nonsteroidal anti-inflammatory drugs piroxicam or sulindac was recently shown to accelerate the development of colitis in interleukin (IL)-10-deficient (IL-10^−/−) mice. Although NSAIDs have been hypothesized to decrease the barrier function of the intestinal epithelium, the mechanism by which this accelerates colitis in IL-10^−/− mice is not well understood. In this study, the effects of piroxicam on the colonic mucosa of IL-10^−/− C57BL/6 mice were evaluated histologically. The effect of piroxicam on intestinal epithelial cells in vitro was assessed using colorimetric and fluorescent assays for cell viability and apoptotic cell death. Interactions of intestinal bacteria with the colonic mucosa were evaluated by rRNA-directed fluorescence in situ hybridization. In vivo treatment of C57BL/6 IL-10^−/− mice with oral piroxicam markedly enhanced apoptosis of colonic epithelium and resulted in focal erosion of the mucosal surface, enhanced bacterial adhesion and invasion, and accelerated the development of colitis. In vitro, piroxicam induced apoptosis of CT26 murine intestinal epithelial cells in a dose-dependent fashion. Piroxicam-induced apoptosis of CT26 cells could not be prevented by addition of exogenous IL-10; however, IL-10 did significantly enhance their rate of proliferation. Thus, exposure to piroxicam enhances intestinal epithelial apoptosis both in vitro and in vivo and facilitates adhesion and invasion of intestinal bacteria into mucosal tissues in vivo. The role of IL-10 in this process requires further study. These studies support the hypothesis that increased exposure of mucosal cells to intestinal bacteria may lead to development of intestinal inflammation in IL-10^−/− or other genetically susceptible individuals.