A significant number of studies indicate that early year preschool programs lead to positive long-term effects. Systematic quality improvement of early year preschool may enhance these outcomes. The ASP Program was built on this principle. In this program preschool staff are supported in their efforts to critically reflect on current practices and to change these. A randomized controlled study was carried out in Denmark from September 2006 to May 2008. The study encompassed 2323 children in 59 preschools in two municipalities. Children were assessed using the Strength and Difficulties Questionnaire at the start of the intervention, at mid term, and by the end. The results indicate that, in the intervention group, children developed fewer emotional symptoms, conduct problems, became less hyperactive and were more attentive. The effect sizes ranged between 0.15 and 0.2.