Helicobacter pylori infection is considered to be a risk factor for gastric cancer. A high prevalence of H. pylori infection and high gastric-cancer incidence are characteristic of the Estonian population. To evaluate the relationship between these 2 events, we studied the seroprevalence of H. pylori in gastric cancer patients (n = 182) and in healthy blood donors (n = 306). A relative anti-H. pylori IgG antibody activity, as detected by ELISA and immunoblot patterns, was correlated with age, stage of the disease and tumor morphology. A significantly higher H. pylori seroprevalence was found in patients in the early stages of tumor development compared with both advanced cancer patients and controls. No significant difference in H. pylori seroprevalence between patients with the intestinal and diffuse types of tumor growth was observed. A decline in the recognition of putatively cross-reacting (33-66 kDa) antigens was noted in the cancer group. The response to vacuolating toxin-related 85-kDa and CagA 120-kDa protein antigens was not altered and was observed more often in the younger group of cancer patients.