Aspirin causes gastroduodenal erosions and/or ulcers in man when taken for prolonged periods. The effects of shorter periods of aspirin, Bufferin, or paracetamol (acetaminophen) intake as used for self-medication are unknown. In a four way, crossover, blinded endoscopic study, we compared the effects of aspirin, Bufferin, paracetamol, and placebo, two tablets four times a day for 24 hours, on the gastroduodenal mucosa of 10 normal volunteers. Both regular aspirin and bufferin produced multiple gastric (p less than 0.005) and duodenal erosions (p less than 0.05, compared with baseline and placebo studies). Paracetamol did not cause significant gastric or duodenal mucosal damage. Two subjects developed duodenal ulcer-like lesions in the course of the study. We conclude that the use of unbuffered aspirin and Bufferin, but not paracetamol, in recommended doses for one day causes significant gastroduodenal mucosal damage.