Background/Aims/Methods: To determine the frequency of Helicobacter pylori infection (Hp-I) in 73 patients with myelodysplastic syndromes (MDS) and 40 controls, serologic analyses of Hp and 13C-urease breath tests (INFAI) were performed. Gastric mucosal biopsy specimens were obtained to determine the presence of Hp-I using a rapid urease test, i.e. the Campylobacter-like organism (CLO) test, and cresyl violet staining. Peripheral blood (PB) flow cytometry for CD3, CD4, CD8, CD14, CD19 and CD34 was conducted in 35 patients and in controls. Results:Hp-I was detected by: (a) serology in 75.34% of patients (p = 0.000), (b) INFAI in 57.69% of patients, (c) CLO in 60.71% of patients and (d) histological confirmation in 80.36% of patients (p = 0.001). No correlation between Hp-I and CD3, CD4, CD8, CD14, CD19 expression, leukemic transformation or death was observed. However, in 20 cases, significant variation in the PB lymphocytic proportion possibly attributable to Hp-I was ascertained, in contrast to the expected MDS ratio. Conclusion: Although there is no evidence for a causal relationship between Hp-I and MDS, the increased prevalence of Hp-I among the MDS patients is an interesting finding that deserves further investigation as it may indicate a common factor causing susceptibilities to both MDS and Hp-I or that Hp might influence the pathophysiology of MDS.