The relationship between Helicobacter pylori infection and idiopathic thrombocytopenic purpura (ITP) has been investigated in several studies. We investigated the prevalence of H. pylori infection and the clinical effects of eradication in 22 Japanese patients with chronic ITP. H. pylori infection was found in 14 (63.6%) of the patients by histologic and culture examinations of biopsy samples obtained by gastrointestinal endoscopy. H. pylori was eradicated by proton pump inhibitors and 2 kinds of antibiotics in 13 (92.9%) of the 14 patients in whom the results of treatment could be evaluated. Five (38.4%) of those 13 patients had platelet recovery (platelet count of more than 100 x 10(9)/L and an increase of more than 30 x 10(9)/L with respect to the baseline value) after eradication. The median follow-up period was 15 months. One patient who had a complete response had a partial relapse after cessation of prednisolone treatment without any evidence of H. pylori reinfection. Another patient, in whom H. pylori was not eradicated even after 2 treatment sessions, had a partial response after treatment. A screening examination for H. pylori infection may be necessary for Japanese patients with newly diagnosed ITP. Although the exact mechanism underlying platelet recovery after H. pylori eradication is not clear, the results of this study indicated that H. pylori eradication treatment is a good option for some patients with chronic ITP.