The relation of infant feeding practices, cigarette smoke exposure, and group child care to the onset and duration of otitis media with effusion (OME) was evaluated in a cohort of 698 healthy infants prospectively monitored by tympanometry in the home every 2 to 4 weeks until 2 years of age. Except for an experimental group of children who were offered early tube placement, the study children received conventional care from their personal physician or clinic. We used LISREL, a structural equation modeling procedure (computer software), to explore associations between environmental variables and OME onset and duration while controlling for interrelations among the variables. Supine feeding position and early initiation of group child care were associated with earlier onset of OME. Shorter duration of breast-feeding, increased packs of cigarettes smoked per day in the home, and increased hours per week in group child care were associated with an increase in the amount of time with OME during one or more of the age blocks studied (birth to 6, 6 to 12, 12 to 18, and 12 to 24 months). For a decrease in the amount of time with OME during the first 2 years of life, prolonged breast-feeding and upright feeding position should be encouraged, and cigarette smoke exposure should be minimized. Limiting early child care in large groups might also be advisable.