The relationship between respiratory infectious diseases and form of day care was assessed in a retrospective cohort study of 2568 randomly selected children aged 1 through 7 years in Espoo, Finland. Day-care center children had an increased risk for the common cold, acute otitis media, and pneumonia. The risk concentrated in 1-year-old children, for whom the adjusted relative risks (incidence density ratios) for the common cold, otitis media, and pneumonia were 1.69 (95% confidence interval [CI] = 1.43, 2.01), 1.99 (95% CI = 1.57, 2.52), and 9.69 (95% CI = 2.31, 40.55), respectively. Among 1-year-old children, the proportion of infections attributable to care at day-care centers were 41% (95% CI = 30, 50) for colds, 50% (95% CI = 36, 60) for otitis media, and 85% (95% CI = 57, 98) for pneumonia. The results provide evidence that care in day-care centers is a determinant of acute respiratory infections in children under 2, whereas family day care does not essentially increase risk.