Background & Aims: Nonsteroidal anti-inflammatory drugs (NSAIDs) cause lower gastrointestinal (GI) clinical events such as bleeding. Cyclo-oxygenase (COX)-2 selective inhibitors decrease upper GI events, but no prospective trial has prespecified assessment of lower GI clinical events. Methods: Patients 2:50 years old with osteoarthritis or rheumatoid arthritis were randomly assigned to etoricoxib (60 or 90 mg qd) or diclofenac (150 mg qd). Lower GI clinical events, confirmed by a blinded adjudication committee, included perforation or obstruction requiring hospitalization or bleeding (gross or occult rectal bleeding without upper GI cause associated with hypotension, orthostatic changes in heart rate [>20 beats per minute] or blood pressure [ > 20 mmHg systolic or > 10 mmHg diastolic], hemoglobin drop >= 2 g/dl, or transfusion; or observed active bleeding or stigmata of hemorrhage). Results: We enrolled 34,701 patients with mean duration of therapy of 18 months. Rates were 0.32 and 0.38 lower GI clinical events per 100 patient-years for etoricoxib and diclofenac (hazard ratio [HR] = 0.84; 95% confidence interval [CI] , 0.63-1.13). Bleeding was the most common event (rates of 0.19 and 0.23 per 100 patient-years, respectively). Multivariable analysis revealed significant risk factors to be prior lower GI event (HR = 4.06; 95% CI, 2.93-5.62) and age >= 65 years (HR = 1.98; 95% CI, 1.45-2.71). Conclusions: A statistically significant decrease in lower GI clinical events was not seen with the COX-2 selective inhibitor etoricoxib versus the traditional NSAID diclofenac. The risk of a lower GI clinical event with NSAID use seems to be constant over time, and the major risk factors are a prior lower GI event and older age.