To evaluate the efficacy of serologic test in diagnosis of Helicobacter pylori infection, ELISA was used to detect IgG antibody against partially purified H. pylori outer membrane antigen (Bio-RAD GAP test IgG). Endoscopic gastric antral biopsy specimens were also obtained for microscopic examination of the bacteria and rapid urease test(CLO test). Eighty one patients (47 nonulcer dyspepsia patients, 11 benign gastric ulcer patients and 23 duodenal ulcer pa- tients) were enrolled in this study. Detection of H. pylori by microscopy was considered as true positive for H. pylori infection. Absence of H. pylori by microscopy and positive CLO test was considereed as equivocal and these cases were excluded in the analysis of the data. H. pylori infections were detected in 58(77%) of 81 patients, 30(64%) of 47 nonulcer dyspepsia pa- tients, 9(82%) of 11 benign gastric lucer patients and 19(83%) of 23 duodenal ulcer pa- tients). Nineteen of 81 were negative and 4 of 81 were equivocal in H. pylori infection. The sensitivity of the serologic test was 72% and the specificity was 53%. In conclusion, the sensi- tivity and specificity of the serologic test for diagnosis of H. pylori infection is very low, they must be improved to be used clinically. (Korean J Gastroenterol 1994; 26: 631 636)