Acute respiratory illness is one of the leading causes of childhood mortality in developing nations. We investigated an increase in hospitalizations of children less than 2 years old for severe lower respiratory tract illness (LRI) from October 1986 through December 1986 on the island of American Samoa. Hospitalization rates were highest in children less than 6 months of age (60 of 1000 compared with 22 of 1000 for those 6 to 11 months of age, and seven of 1000 for those 12 to 24 months of age). The outbreak was more severe than in previous years, with ten (19%) of 54 patients admitted to the intensive care unit (ICU) compared with three (7%) of 42 and one (3%) of 40 during the same months of 1985 and 1984. Two patients died. Most patients had clinical bronchiolitis; of 34 patients tested, serologic or nasal aspirate evidence of recent respiratory syncytial virus (RSV) infection was found in 18 (53%). A study of patients younger than 1 year of age demonstrated that patients hospitalized with LRI were more likely to have a household member who smoked cigarettes (18/20, 90%) than outpatient controls without recent respiratory illness (8/15, 53%). Maternal sera obtained between December 1985 and October 1986 showed no protective effect of either detectable IgG or neutralizing antibody to two major groups of RSV. Our study documents the involvement of RSV in an outbreak of severe LRI among infants in a tropical area and further suggests that exposure to cigarette smoke is a risk factor for LRI infants.