Using data from the Head Start Impact Study (n = 3,696), this article examines whether one year of Head Start differentially benefited parents as a function of their initial parenting behaviors. Four outcomes are examined, namely, parents' rates of engaging in cognitive stimulation, reading to their child, and spanking, as well as their depressive symptoms. In general, most parents demonstrated improvements in their reading practices and cognitive stimulation regardless of their parenting behaviors at baseline. However, depressive symptoms and spanking behavior showed improvements only among parents who began the Head Start program with the most depressive symptoms and the most frequent spanking, respectively. These findings suggest that treatment-induced changes in parenting can vary by parents' incoming attributes and that heterogeneity of effects should be considered. Implications for Head Start and other parenting interventions are discussed. (author abstract)