The aim of this double-blind endoscopic study was to compare the effects of placebo (group I, 5 patients), lysine acetylsalicylate (group II, 7 patients) and acetylsalicylic acid (group III, 7 patients) on the gastric and duodenal mucosa in healthy humans. Endoscopy was performed before and one hour after endoscopic instillation of aspirin (500 mg) or placebo in the stomach. Endoscopy was repeated after one week of aspirin-treatment (2 g per day) or placebo. Endoscopic findings were graded from 0 to 6 with regard to the aspect of the lesions (petechiae, erosions, ulcers) and to their number (less than 10; greater than 10). One hour after placebo instillation endoscopic findings were normal in all the patients of group I. Three and 5 patients of groups II and III, respectively, developed gastric lesions but none had duodenal lesions. At day 8 only one subject from group I had gastric petechiae. After one week of aspirin-treatment, 13 out of the 14 subjects of groups II and III developed gastric lesions and 3 in each group had duodenal lesions. The endoscopic score was significantly higher in group III than in group II for the following localisations: fundus, antrum, entire stomach, and stomach + duodenum. However the duodenal score was not significantly different between these 2 groups. It is concluded that, after a one-week treatment in normal patients, standard aspirin produces 2 fold more gastric mucosal damage than does soluble aspirin.