We investigated extensively an outbreak of hepatitis A at a factory in suburban Nagoya. Epidemiological study indicated a foodborne outbreak by a supplier of lunches. Serologically, all the employees younger than 30 years of age had been susceptible to hepatitis A virus, but the highest morbidity was observed in the 40-44 age group. The age difference in morbidity from foodborne hepatitis and susceptible populations suggests a shift in mean patient age linked to a shift in antibody prevalence to hepatitis A virus. In communities where the prevalence started shifting after development of sanitary systems, effective prophylaxis for foodborne hepatitis A will be necessary to prevent the disease in an increasing number of older patients in a few decades.