We prospectively assessed the relations between various characteristics of day care and lower respiratory illness (LRI) in a cohort of 1,268 Minnesotan children, born between October 1989 and January 1991 and followed to 2 yr of age. Information on LRI was abstracted from medical records and data on day care use, respiratory symptoms, and physician diagnosis of asthma were obtained from questionnaires. We identified a subgroup of 60 children with recurrent wheezing illnesses. The LRI rate ratio for day care attendance was 2.0 (95% confidence interval = 1.7, 2.2). Rate ratios were similar regardless of the day care setting, number of other children present, or the number of hours spent in day care. A parental history of asthma further increased the rate ratio for day care attendance. Day care attendance was associated with a threefold risk of having recurrent wheezing illnesses. We conclude that day care attendance is an important risk factor for LRI in young children, and for recurrent wheezing illnesses.