The purpose of this study was to examine the average treatment effect of preschool special education services on children's kindergarten academic skills. Using data from a nationally representative sample of United States children who participated in the Early Childhood Longitudinal Study--Birth Cohort, we examined the effectiveness of preschool special education services by comparing reading and math outcomes for children who received special education services at preschool-age to a propensity-score-weighted sample of children who did not receive these services. Results indicated that the receipt of these special education services had a statistically significant moderate negative effect on children's kindergarten skills in both reading (d=-0.21) and mathematics (d=-0.29). These findings have implications for the implementation and evaluation of services for young children experiencing developmental delays or disabilities. (author abstract)