The present study evaluated the effect of a one-day high-dose combined therapy on Helicobacter pylori infection. Thirty-two consecutive patients (suffering from either peptic ulcer or nonulcer dyspepsia) with Helicobacter pylori infection received omeprazole (40 mg) + bismuth subcitrate (240 mg x 4) + amoxicillin suspension (2000 mg x 4) + metronidazole (500 mg x 4), for only one day. Endoscopy, histology, culture, and susceptibility studies were done at entry and 30 and 90 days after the treatment day. Successful eradication was obtained in 23/32 (72%) patients and gastritis had resolved in 95% of these. Side effects were induced by the treatment in 6/32 (19%) patients, but these were all self-limiting, short-lasting, and did not require any specific treatment. Development of bacterial resistance to metronidazole occurred in 6/9 (67%) non-eradicated patients. These data suggest that one-day treatment with high doses of amoxicillin, metronidazole, bismuth, and omeprazole represents an effective, safe, and inexpensive therapeutic approach for the treatment of H. pylori infection.