Helicobacter pylori is categorized into two phenotypes on the basis of the presence or absence of the CagA protein. CagA protein-positive H. pylori are more closely associated with peptic ulcer disease and cancer. Whether CagA-positive strains are similarly represented among racial or ethnic groups in northern California was investigated. Sera from 152 H. pylori-infected healthy young adults were tested by ELISA for IgG against CagA. CagA antibodies were detected in 79.4% of blacks, 63.8% of Hispanics, and 50% of whites. After adjusting for demographic factors, blacks had significantly more infections with CagA-positive H. pylori than did whites (odds ratio [OR] = 5.0; 95% confidence interval [CI] = 1.6-15.3) or Hispanics (OR = 5.5, 95% CI = 1.9-16.0). Also, there was a higher prevalence of CagA in persons born in developing countries than in persons born in industrialized nations (OR = 3.5, 95% CI = 1.3-9.4). This suggests either a genetic predisposition of racial or ethnic groups to infection with particular H. pylori phenotypes or transmission of H. pylori within relatively segregated population groups.