OBJECTIVE: Child care outside the home increases children's infections substantially. We have to evaluate the possibilities for reducing the transmission of infections by an infection prevention program. DESIGN AND METHODS: A 15-month randomized controlled trial involving 20 day-care centers was conducted to evaluate the efficacy of an infection prevention program. The program was introduced in 10 centers and the other matched 10 centers served as controls. Records were made of the occurrence of infections and absences from care or work because of infections among the children, their parents and the personnel of the day-care centers. RESULTS: Both the children and the personnel in the program centers had significantly fewer infections than those in the control centers, the reduction being 9% [95% confidence interval (95% CI), 4 to 16%, P