Serological studies in developed and developing countries using enzyme-linked immunosorbent assays have validated this technique as a rapid, noninvasive method for the diagnosis of Helicobacter pylori infections. The prevalence of serum antibodies to H pylori was studied in 473 Canadian blood donors from Manitoba, 212 healthy Japanese. and 226 healthy Americans. As expected, the seroprevalence rose progressively with age in the three populations and reached its peak (greater than 55%) in subjects 60 years of age and older. The seroprevalence did not decrease in elderly persons (60 to 99 years), indicating a persistent immune response. More detailed analysis was perfom1ed on the Canadian population. Age-adjusted prevalence rates in men and women were similar. Among young adults (15 to 29 years). farmers had a significantly higher seroprevalence rate than white-collar or blue-collar workers. but in older persons occupational rates were similar. A multiple linear regression analysis of the data confirmed that age and occupation in young adults were both significantly associated with seroprevalence of H pylori infections.