Helicobacter pylori eradication is useful for improvement of a half of patients with idiopathic thrombocytopenic purpura (ITP), but its long-term therapeutic efficacy has not been elucidated. We investigated the long-term efficacy of H. pylori eradication in 30 cases with ITP that were included in our previous study regarding the association between H. pylori infection and ITP. Twenty-one cases were positive and nine cases were negative for H. pylori infection. H. pylori eradication therapy including secondary regimen was successful in 20 cases, half (responder) of whom showed ITP remission 1 month later. Nine responders could be followed up for a long time and did not show re-infection of H. pylori. Eight of nine needed no medication except for eradication therapy. Another case remained in remission for 1 year but thereafter needed a steroid therapy due to the recurrence. Eight nonresponders could be followed up for a long time. All these cases showed a bad clinical course even though they received the other post-treatments including steroid therapy. Three of nine H. pylori-negative cases underwent eradication therapy after obtaining the written informed consent, but none of them showed improvement. Of these three cases, two cases could be followed up. Only one case remained a remission although receiving corticosteroid as a post-treatment. Conditions of H. pylori-negative ITP cases were usually unstable for a long time. H. pylori eradication has a short-term efficacy for about half of H. pylori-positive ITP patients, and the responders to the eradication therapy may receive a long-term clinical benefit without other therapies.