Sixty volunteers were endoscopically evaluated to compare gastric mucosal injury following oral administration of sulindac, naproxen, aspirin, or placebo for two consecutive seven-day periods. A single-blind technique was utilized wherein the endoscopist was unaware which drug each volunteer had received. The following dosages were employed for the two study periods: sulindac, 150 and 200 mg, b.i.d., naproxen, 250 and 375 mg, b.i.d., and aspirin, 650 and 975 mg, q.i.d. The only subject who developed a frank ulcer with mucosal bleeding was in the sulindac group, however volunteers taking sulindac demonstrated statistically less significant mucosal injury on endoscopic examination than those receiving naproxen or aspirin.