Summary. The prevalence of Helicobacter pylori infection and the effect of its eradication on platelet count in 48 Japanese patients with autoimmune thrombocytopenic purpura (AITP), including 40 chronic idiopathic thrombocytopenic purpura (ITP) and eight secondary AITP, were investigated. H. pylori infection was found in 25 ITP patients (62·5%) and in two secondary AITP (25%). H.pylori eradication was obtained in 19 of 19 infected ITP patients (100%), who were not in remission (platelets < 100 × 109/l) at the time of infection assessment. During follow-up (median 14·8 months), 12 of 19 H. pylori-eradicated patients (63·2%) showed a significant increase in platelet count accompanied by a significant decrease of platelet-associated immunoglobulin G (IgG). This response was maintained in all responding patients throughout the follow-up period. However, two infected patients with secondary AITP did not show platelet increase after eradication. The assessment of H. pylori infection and its eradication should be attempted in ITP as this approach could be an effective strategy, at least for some of these patients.