Data from the Head Start Impact Study (N = 3,185, age = 3-4 years) were used to determine whether 1 year of Head Start differentially benefited children from homes with high, middle, and low levels of parental preacademic stimulation on three academic outcome domains--early math, early literacy, and receptive vocabulary. Results from residualized growth models showed positive impacts of random assignment to Head Start on all three outcomes, and positive associations between parental preacademic stimulation and academic performance. Two moderated effects were also found. Head start boosted early math skills the most for children receiving low parental preacademic stimulation. Effects of Head Start on early literacy skills were largest for children receiving moderate levels of parental preacademic stimulation. Implications for Head Start are discussed.