From November 1989 through March 1990, five cases of serologically confirmed acute hepatitis B (HB)* among American Indians from two Montana Indian reservations (combined population: 6300) were reported by Indian Health Service (IHS) clinic staff to the Billings IHS Area Office. In comparison, during 1986-1988, an average of six HB cases among American Indians were reported annually among persons residing both on reservations and statewide (1990 population of American Indians in Montana: 47,679). Four of the five persons with acute HB reported histories of injecting-drug use (IDU).