Head Start programs currently provide early childhood education and family support services to more than 900,000 low-income children and their families across the United States with an annual budget of around $8 billion in state and federal funds. Researchers and policy makers have debated the program's effectiveness since its inception in 1964. Dozens of studies (see for example Currie & Thomas, 1993; Deming, 2009; Ludwig & Miller, 2007) over the last four decades have provided a mixed picture of results with some showing strong positive impacts and others indicating small or null effects. In an effort to resolve these long standing disputes over the impact of Head Start, the Administration for Children and Families commissioned a random assignment study of Head Start programs in 2000. To date, the results of the Head Start Impact Study (HSIS) have done little to settle debates about the effects of Head Start attendance on children's development. Some have suggested that the HSIS results may be attributable, in part, to the fact that more than one-third of children assigned to the study control group enrolled in some form of early childhood education that offered services similar to those provided by Head Start (National Forum on Early Childhood Policy and Programs, 2010). There is some empirical support for this hypothesis. However, unpacking treatment effect by variation in control group setting is difficult using traditional methods. The standard Two-Stage Least Squares (TSLS) framework can be used to estimate effects of Head Start relative to no Head Start among students who are "compliers" with the randomization scheme. Yet, there are many other instances of uncontrolled variation that are of policy interest in the context of HSIS. Given this variation, an important question to ask is whether enrollment in Head Start by virtue of the randomized offer to do so has differential effects according to the alternate care type setting in which a child would otherwise be. In this paper, the authors extend the main HSIS findings to examine whether the offer of Head Start was differentially beneficial for students grouped by the care setting they would have received absent the offer of Head Start. Tables and figures are appended.