It is well known that blood group antigens are related to the development of peptic ulcer and gastric carcinoma. This study sought to determine the relationship between H. pylori and ABO/Rhesus blood groups, age, gender, and smoking. Patients (335 women and 205 men; mean age, 51.68 +/- 15.0 years; range, 18-85 years) who attended our outpatient clinic were enrolled in the study. All patients were randomly selected in each age group. Demographic data recorded for each patient included age, gender, and tobacco use. Blood samples were tested for H. pylori antibodies, and ABO/Rhesus blood group antigen typing was performed. Serum antibodies were tested against H. pylori infection. Prevalences of all blood groups were O (29.2%), A (38.2%), B (17.8%), and AB (14.8%). As expected from previous studies, we found that seropositivity for H. pylori increased with age. H. pylori Ig G antibody positivity was detected in 185 of 335 women (60.6%), compared with 88 of 205 men (42.9%), a statistically significant difference (P < 0.05). H. pylori Ig G antibody positivity was detected in 206 of 379 nonsmokers (54.3%) compared with 67 of 161 smokers (41.6%), a statistically significant difference (P < 0.05). Patients in blood groups A and O were more prone to H. pylori infection than were patients in other blood groups (P < 0.05), and patients in the AB blood group were less prone to H. pylori infection compared with patients in other blood groups (P < 0.05). The results of this study demonstrate that H. pylori infection can be related to ABO blood group, age, gender, and smoking.