Patients (454) referred for gastroscopy to the General Hospital of Athens were examined to determine (1) if non-secretors were over-represented among patients with ulcers and (2) if there was an association with ABO blood group or secretor status and carriage of Helicobacter pylori. Compared with the local population, among patients with either gastric ulcer (51) or duodenal ulcer (96) there was a significant increase in the proportion of those who were blood group O (P less than 0.025); however, there were no significant differences in the proportions of non-secretors. H. pylori was identified in 62% of the 454 patients: 59.5% of those without evidence of ulcers; 62.5% of those with gastric ulcer; 88% of those with duodenal ulcer (P less than 0.0005). These bacteria were cultured more often and in higher numbers from patients with duodenal ulcer (P less than 0.025). There was no association between ABO blood group and prevalence of H. pylori. The prevalence of H. pylori among non-secretors with gastric ulcer (12.5%) was significantly lower than that for non-secretors with duodenal ulcer (100%) (P less than 0.0005). This was not observed for secretors.