Research on the effects of preschool dosage on children's early academic functioning has been limited despite the substantial policy implications of such work. The present study adds to a growing literature on this topic by examining how the number of years enrolled in an enhanced preschool program impacts the school readiness of primarily low-income children at kindergarten. Multi-level modeling was used to account for nesting of children within classrooms. To control for potential selection bias since children were not randomly assigned to receive one or two years of preschool, propensity score one-to-one matching was used to create the two participant groups. Receiving a second year of preschool led to significant improvements in children's early literacy and numeracy skills. Implications of these results for preschool interventions are discussed. (author abstract)