To determine the prevalence of Helicobacter pylori (H pylori) infection in North American patients with immune thrombocytopenic purpura (ITP) and the effect of H pylori eradication on the platelet count, a prospective study was performed. Seventy-four patients aged 10 years and older (mean age of 41 years) with chronic ITP and a platelet count below 60 x 10(9)/L were enrolled. H pylori infection was found in 22% of patients by means of a breath test and could not be predicted by gastrointestinal symptoms. H pylori-positive patients (52.5 years of age) were older than H pylori-negative patients (38.5 years of age; P =.0035). Fifteen of the 16 H pylori-positive patients were treated and the bacteria was eradicated in 14 (93%). After 3 months, a significant response (platelet count > 50 x 10(9)/L and doubling the initial count) was observed in only one patient. After a median follow-up of 11.5 months, none of the 14 patients had responded. Ten H pylori-negative patients treated with the same regimen also did not increase their platelet counts. In conclusion, unlike several previous reports, this study does not implicate H pylori in the pathogenesis of ITP since the prevalence of H pylori infection was low and eradication of H pylori did not positively influence the course of the ITP.