Consistent evidence that the effect of preschool intervention on cognitive achievement fades with the passage of time has resulted in a search for mediators of preschool. This study investigated factors that play a role in mediating the effects of a government funded Child-Parent Center preschool program. The school adjustment of 266 low-income, mostly Black preschool children and of 125 comparison group children were matched on neighborhood characteristics and were traced from kindergarten through the third year of school (1986-1989). Data were collected from children, parents, and teachers on entering kindergarten cognitive readiness, teacher ratings of socioemotional maturity, parental involvement at home and in school, grade retention, assignment to special education, school mobility and cognitive achievement in reading and mathematics. Results of a latent-factor structural model indicated that preschool influenced later achievement and retention indirectly rather than directly. Four major pathways through which preschool exerted its effect included (1) cognitive readiness, (2) cognitive readiness and teacher ratings of socioemotional maturity, (3) teacher ratings of socioemotional maturity, and (4) parent involvement and school mobility. Cognitive readiness, teacher ratings, and parent involvement also transmitted effects to grade retention. That preschool's influence on later outcomes is largely indirect indicates its dependency on intervening factors in exerting effects. These intervening factors appear to be critical in promoting school success of children at risk.