OBJECTIVES. It is well known that children attending childcare have a higher risk of acute respiratory infections compared with children in home care; however, knowledge is sparse regarding how the excess risk of acute respiratory infection varies with age, time since enrollment, and other factors. METHODS. A national register-based study of 138821 inpatient admissions to hospital for acute respiratory infection during 3982925 person-years of follow-up in Danish children aged 0 to 5 years. Data on child and family characteristics, childcare attendance, and hospitalizations were obtained from Danish registries. The outcome of the study was inpatient admissions to hospital for acute respiratory infection. Incidence rate ratios were estimated using Poisson regression. RESULTS. In children <1 year of age, the first 6 months of enrollment in the first childcare facility were associated with a 69% higher incidence of hospitalizations for acute respiratory infection compared with children in home care. Similar figures for children aged 1, 2, and ≥3 years were 47%, 41%, and 8%, respectively. The incidence decreased after the first 6 months, and after ≥1 year in childcare the incidence was comparable with that of children in home care. Similar patterns were seen after second enrollment. For 0- to 2-year-old children living in households with no additional children <5 years, the excess incidence during the first 6 months of enrollment was 100% compared with 25% and 9% for children living with 1 and ≥2 additional children, respectively. CONCLUSIONS. The increased risk of acute respiratory infection was most pronounced among 0- to 2-year-old children living with no other children during the first 6 months of enrollment. Our findings may suggest that it would be optimal to postpone enrollment into childcare until after 1 year of age.