A small epidemic of nursery-associated hepatitis A (HA), in which silently infected children played a cryptic role in transmitting the illness to adult contacts, was investigated. It was found that a male nurse had transmitted HA to four children before he fell ill. He was not heard from for about two months as he was a part-time worker and was admitted to a local hospital in another prefecture. Two of the four silently infected children transmitted HA to their parents who then revealed the presence of nursery-associated HA. The epidemic was terminated by injection of human gamma globulin to nursery children and to antibody-negative parents, nursery staff and other contacts except for two breakthrough cases.