We investigated the relationship between the severity of Helicobacter pylori-associated gastritis and serum anti-H. pylori antibodies. A total of 158 consecutive patients underwent upper gastroduodenal endoscopy. At the time of endoscopy, three biopsy specimens were taken from the antrum and the body of the stomach for culture of H. pylori, CLO test, and histology. Specimens with a negative rapid urease test and no organisms found by culture were judged to be H. pylori-negative. The GAP test used for the antibody measurement is a kit designed to measure serum IgG, IgA, and IgM antibodies. The severity of gastritis was classified according to the Sydney System. The sensitivity and specificity of the IgG antibody measurement kit was 98.3 and 71.4%, respectively. The sensitivity and specificity of the IgM antibody measurement kit was 14.3 and 60.8%, respectively. The sensitivity and specificity of the IgA antibody measurement kit was 45.1 and 100%, respectively. Serum anti-H. pylori IgG antibody titers were correlated significantly with the severity of inflammation in both the antrum and body. Significant associations were found between serum anti-H. pylori IgG and IgA antibody titers and the development of atrophic gastritis. These results suggest that measurement of serum anti-H. pylori antibody titers is useful in the diagnosis of H. pylori infection and severity of gastritis.