Helicobacter pylori has recently been postulated to play a role in the pathogenesis of autoimmune diseases, including idiopathic thrombocytopenic purpura (ITP). We investigated the prevalence of H pylori infection and the effects of its eradication in 61 patients with ITP. H pylori infection was found in 50 patients (83%), an incidence significantly higher than not only healthy volunteers in Japan (60%) but also subjects in other reported ITP series (approximately 43%-71%). In our study, the mean age of H pylori-positive ITP patients (58.0 years) was significantly higher than that of H pylori-negative ITP patients (40.5 years). Bacterium eradication efforts were performed in 29 infected ITP patients and succeeded in 27 patients (93%). The 29 patients with eradicated H pylori infections showed significant increases in platelet counts compared with patients with uneradicated infections or who were H pylori-negative. During the follow-up period (median, 11.0 months), 16 (55%) of 29 patients achieved a major or a minor response. The patients who achieved a major response had not received previous prednisolone therapy, suggesting a relationship between prednisolone therapy and the response to eradication efforts. The assessment of H pylori infection and its eradication should be attempted in cases of ITP, because this approach may be a good new strategy for treating some ITP patients, especially elderly Japanese patients. Some regional factors have been suggested as causes of H pylori-associated ITP.