BACKGROUND: Antimicrobial resistance has decreased eradication rates for Helicobacter pylori infection worldwide. OBJECTIVE: To determine whether sequential treatment eradicates H. pylori infection better than standard triple-drug therapy for adults with dyspepsia or peptic ulcers. DESIGN: Randomized, double-blind, placebo-controlled trial. SETTING: Two Italian hospitals between September 2003 and April 2006. PATIENTS: 300 patients with dyspepsia or peptic ulcers. MEASUREMENTS: (13)C-urea breath test, upper endoscopy, histologic evaluation, rapid urease test, bacterial culture, and assessment of antibiotic resistance. INTERVENTION: A 10-day sequential regimen (40 mg of pantoprazole, 1 g of amoxicillin, and placebo, each administered twice daily for the first 5 days, followed by 40 mg of pantoprazole, 500 mg of clarithromycin, and 500 mg of tinidazole, each administered twice daily for the remaining 5 days) or standard 10-day therapy (40 mg of pantoprazole, 500 mg of clarithromycin, and 1 g of amoxicillin, each administered twice daily). RESULTS: The eradication rate achieved with the sequential regimen was significantly greater than that obtained with the standard treatment in the intention-to-treat analysis (89% vs. 77%; P = 0.0134; difference, 12% [95% CI, 3% to 20%]), the modified intention-to-treat analysis (91% vs. 78%; P = 0.0022; difference, 13% [CI, 5% to 21%]), and the per-protocol analysis (93% vs. 79%; P = 0.0013; difference, 14% [CI, 6% to 21%]). Sequential therapy was significantly more effective in patients with clarithromycin-resistant strains (89% vs. 29%; P = 0.0034). The incidence of major and minor side effects did not differ between therapy groups (17% in both groups). One patient (0.7%) in the standard therapy group discontinued treatment because of side effects. LIMITATIONS: Follow-up was incomplete in 4.6% and 2.7% patients in the sequential therapy and standard therapy groups, respectively. The results may not be generalizable to other countries. Sequential therapy may be more effective because it includes 1 additional antibiotic (tinidazole) that is not contained in standard therapy. CONCLUSIONS: Sequential therapy is statistically significant compared with standard therapy for eradicating H. pylori infection and is statistically significantly more effective in patients with clarithromycin-resistant strains. Side effects are similar with both treatment regimens and are rarely severe enough to cause discontinuation of therapy. ClinicalTrials.gov registration number: NCT00403364.