This study measures the effects of Head Start enrollment on multiple school readiness outcomes of a nationally-representative sample of nearly 5,000 low-income children. This randomized and controlled trial compares the cognitive and social-emotional development, as well as select academic, literacy, health, and behavior-related outcomes of groups of children either attending Head Start or not attending Head Start. Follow-up data was collected at either kindergarten or 1st grade from of cohorts of children who experienced a year of Head Start either at either age 3 or age 4. Analysis revealed that access to Head Start has benefits for both cohorts in the cognitive, health, and behavioral areas. Social-emotional benefits occurred for 3-year-olds only. Overall, however, the benefits of access to Head Start at age four tend to fade by first grade except in select sub-groups of children.