The efficacy of omeprazole in the elimination of Helicobacter pylori was investigated in a prospective randomized-controlled trial. 50 patients with upper gastrointestinal symptoms and chronic active H. pylori-associated gastritis were allocated to one of the following four therapeutic schedules: 1) omeprazole 40 mg/d for 4 weeks (n = 13); 2) bismuth subsalicylate (BSS) 3 x 600 mg for 4 weeks (n = 12); 3) omeprazole plus BSS for 4 weeks (n = 13); 4) triple therapy (BSS for 4 weeks, amoxicillin 3 x 750 mg and metronidazole 3 x 400 mg for 10 days) (n = 12). Clinical symptoms, endoscopic and histologic findings, and H. pylori status were reassessed immediately after therapy, and 1 and 6 months later. After cessation of therapy bacterial clearance rates were: 1) omeprazole 2/13 (15%); 2) BSS 6/12 (50%); 3) omeprazole plus BSS 5/13 (38%); 4) triple therapy 10/12 (83%). The degree of density of gastric mucosal infestation with H. pylori and the degree of activity of gastritis was reduced in all treatment groups but was most prominent after triple therapy. Clinical symptoms improved in all treatment groups. One and six months after completion of therapy H. pylori eradication rates were: 1) omeprazole 0/13 (0%); 2) BSS 1/12 (8%); 3) omeprazole plus BSS 1/13 (8%); 4) triple therapy 10/12 (83%). Our study shows that 40 mg/d omeprazole is ineffective in eradicating H. pylori. Dual therapy with omeprazole and bismuth subsalicylate does not improve bacterial elimination. Only triple therapy effectively eradicates H. pylori.