To determine the HLA-DQ locus in Mexican patients with Chronic gastritis and gastric adenocarcinoma. Oligotyping for HLA-DQ locus was performed in 45 Mexican patients with chronic gastritis and 13 Mexican patients with diffuse-type gastric adenocarcinoma, and was then compared with 99 clinically healthy unrelated individuals. H pylori infection and CagA status were assessed in patients by enzyme-linked immunosorbent assay (ELISA) method. We found a significant increased frequency of HLA-DQB1*0401 allele in H pylori-positive patients with chronic gastritis when compared with healthy subjects [19 vs 0%, P = 1 x 10(-7), odds ratio (OR) = 4.96; 95% confidence interval (95% CI), 3.87-6.35]. We also found a significant increased frequency of HLA-DQB1*0501 in patients with diffuse-type gastric carcinoma in comparison with healthy individuals (P = 1 x 10(-6), OR = 13.07; 95% CI, 2.82-85.14). HLA-DQ locus may play a different role in the development of H pylori-related chronic gastritis and diffuse-type gastric adenocarcinoma in the Mexican Mestizo population.