We randomly assigned 45 adult patients with Campylobacter pylori-confirmed antral gastritis to 8 days of treatment with 1 g amoxycillin suspension twice a day, or placebo, according to a double-blind study design. At the end of therapy, 91% of patients treated with amoxycillin demonstrated clearance of the organism from the antrum, compared with 16% in the placebo group (p less than 0.001). Active antral gastritis resolved in 68% of patients in the amoxycillin group versus only 9% in the placebo group (p less than 0.001). No significant difference was observed when looking at the evolution of chronic only gastritis. No significant improvement was observed in the assessment of clinical symptoms and endoscopic appearance. Reappearance of C. pylori and significant worsening of the histological score of active gastritis was observed after 2 wk in all patients. In a second study phase, 18 patients initially not cleared of their bacteria received amoxycillin single blind for 14 days. Clearance of bacteria associated with improvement or resolution of active gastritis was observed in 72% of the cases. In this subgroup, all patients investigated after 1 month were recolonized with C. pylori and, again, had histological active gastritis. We conclude that amoxycillin is effective in treating active antral gastritis associated with C. pylori, but not in preventing relapses, which occur in all cases within 1 month after therapy.