Diseases associated with Helicobacter pylori infection, such as peptic ulcer disease and gastric cancer, afflict men more frequently than women. No study, however, has demonstrated any difference in sex-specific rates of H. pylori infection. In a healthy population undergoing multiphasic health evaluations in 1992-1993 as members of the Kaiser Permanente Medical Care Program of Northern California, adults aged 20-39 years were screened for antibodies to H. pylori infection using a serum enzyme-linked immunosorbent assay and were surveyed with regard to their demographic characteristics and health practices. Among 556 African-American, Hispanic, and white men and women, male sex was a significant risk factor for infection. Other risk factors included African-American race and Hispanic ethnicity, increasing age, living with children, birth in a developing country, and lower levels of income and education. Men consistently had a higher prevalence of antibodies across all strata of race/ethnicity, age, education, and income, and in multivariate analysis male sex remained significantly associated with infection (odds ratio = 2.0, 95% confidence interval 1.2-3.1). African-American race, Hispanic ethnicity, increasing age, lower levels of education, and birth in a developing country were also associated with infection in multivariate analysis. Data from previously reported seroprevalence studies support a tendency for men to have a higher risk of infection. The higher prevalence of infection among young males as observed in Northern California may account in part for the increased incidence of H. pylori-related diseases among men in later decades of life.