To investigate the effect of a high-protein diet on corpus atrophic gastritis in Helicobacter pylori-infected Mongolian gerbils, H. pylori was administered orally to 5-wk-old Mongolian gerbils; and the animals were then fed a control diet (Group C); a high-fat diet (Group F: 40% fat); a high-protein diet (Group P: 32% protein); or a high-fat, high-protein diet (Group FP: 40% fat, 32% protein) for 50 wk beginning at 7 wk of age. In uninfected animals, the mucosal thickness of the corpus was significantly greater in Group P and Group FP than in Group C (P < 0.05). In infected animals, the serum gastrin level was significantly decreased in Group FP and marginally significantly decreased in Group P (P = 0.057) in comparison to Group C. The mucosal thickness of the corpus was significantly greater in Group P and Group FP than in Group C (P < 0.05). Mean inflammation and atrophy scores in the corpus were significantly lower in the high-protein groups (Groups P and FP) than in the control groups (Groups C and F; both inflammation and atrophy: P < 0.05). In conclusion, long-term administration of a high-protein diet suppresses corpus atrophic gastritis in H. pylori-infected Mongolian gerbils.