Since clarithromycin is expected to be widely used to treat Helicobacter pylori infection in the near future, it is important to investigate the relationship between resistance to clarithromycin and the regimens of eradication therapy. We investigated: (1) the usefulness of susceptibility tests prior to eradication therapy, and (2) the rate of acquisition of H. pylori resistance to clarithromycin after treatment failure. Drug susceptibility tests to clarithromycin and amoxicillin were conducted by Dry Plate Test or E-test. The subjects in the first part of this study included 112 patients with H. pylori infection who received triple therapy with various combinations of drugs, including clarithromycin. The eradication rate in patients with clarithromycin-susceptible H. pylori was significantly higher than that in patients with clarithromycin-resistant H. pylori. The second part of this study included 21 patients in whom H. pylori was not eradicated by triple therapy and 12 patients in whom H. pylori was not eradicated with dual therapy including clarithromycin. Of the 33 patients showing non-eradication. 90.9% of those treated with dual therapy and 35.7% of those treated with triple therapy acquired secondary resistance of H. pylori to clarithromycin. We conclude that it is important to conduct drug susceptibility tests prior to treatment of H. pylori infection. Since the incidence of acquiring clarithromycin resistance was significantly higher in the patients showing non-eradication, it is important to choose a regimen with a higher eradication rate, such as triple therapy.