Objective. Helicobacter pylori (H. pylori) infection induces both gastric (GU) and duodenal ulcers (DU). We examined whether host immunological response to H. pylori determines different disease outcomes. Material and methods. Thirty-two GU and 28 DU patients infected with H. pylori, and 24 dyspeptic patients without infection were enrolled. The constituents of cellular infiltrates in biopsies from each patient were determined and lymphokines secreted by stimulated T cells were measured. Serum concentrations of IgG subclasses specific to H. pylori were measured. Results. Low pepsinogen I and high pepsinogen II levels were observed in GU patients, while a high pepsinogen I level was found in DU patients. T cells predominate over other cell types in both GU and DU patients. GU patients had a higher number of T cells (p < 0.01) and lower plasma cells (p< 0.05) than those in DU patients. T cells from GU patients produced greater amounts of IFN-γand less IL-4 than those in DU patients (p < 0.01). GU patients had a higher serum level of IgG2 specific to H. pylori than that in DU patients (p < 0.01). Conclusions. Th response by gastric T cells in GU patient was more polarized to Th1 as compared with that in DU patients, suggesting that a distinct immune response to H. pylori induces different disease outcomes. Helicobacter pylori (H. pylori) infection induces both gastric (GU) and duodenal ulcers (DU). We examined whether host immunological response to H. pylori determines different disease outcomes. Material and methods. Thirty-two GU and 28 DU patients infected with H. pylori, and 24 dyspeptic patients without infection were enrolled. The constituents of cellular infiltrates in biopsies from each patient were determined and lymphokines secreted by stimulated T cells were. Serum concentrations of IgG subclasses specific to H. pylori were measured. Results. Low pepsinogen I and high pepsinogen II levels were observed in GU patients, while a high pepsinogen I level was found in DU patients. T cells predominate over other cell types in both GU and DU patients. GU patients had a higher number of T T cells from GU patients produced greater amounts of IFN-γ and less IL-4 than those in DU patients (p <0.01) 0.01). GU patients had a higher serum level of IgG2 specific to H. pylori than that in DU patients (p <0.01). Conclusions. Th response by gastric T cells in GU patient was more polarized to Th1 as compared with that in DU patients, suggesting that a distinct immune response to H. pylori induces different disease outcomes.