To assess the relation between morbidity from acute diarrhea and the form of day care. The design was a retrospective cohort study. The setting was the city of Espoo, an urban-suburban municipality in southern Finland with a population of 170,000. The study population comprised 2568 randomly selected children aged 1 to 7 years. The main outcome measure was the occurrence of diarrhea. Children in day-care centers (DCCs) had an increased risk for acute diarrhea compared with children in home care. In the whole group of children in DCCs, the relative risk was 1.20 (95% confidence interval [CI], 1.08 to 1.34). The risk was greatest in 1- and 2-year-old children, for whom the estimated relative risks were 1.76 (95% CI, 1.28 to 2.43) and 1.56 (95% CI, 1.16 to 2.09), respectively. The proportion of diarrhea episodes attributable to DCC care in 1-year-old children was 49% (95% CI, 18% to 91%), in 2-year-old children 37% (95% CI, 11% to 73%), and in the whole group 17% (95% CI, 7% to 29%). The infection risk did not differ between children in home and family care. The results provide quantitative evidence that the care in DCCs is a major determinant of acute diarrhea in children, whereas family day care does not increase the infection risk.