It is the impression of many physicians that the incidence of viral hepatitis among Americans abroad is much higher than that in the United States. There is very little information available, however, to assist them in determining whether this is indeed true. Attack rates have only been determined for military populations, and may not apply to civilians who obviously live under very different circumstances. To assess the extent to which viral hepatitis is a risk, a group is needed which has been under medical supervision while residing in countries thought to be "hyperendemic." American missionaries and their families are such a group, and seem to be a suitable population in many respects. They remain abroad for long periods of time, work in close contact with the indigenous populations, and in general have not received immune globulin for prevention of infectious hepatitis. Accordingly, in October 1964 the present study was undertaken.