In policy research a frequent aim is to estimate treatment effects separately by subgroups. This endeavor becomes a methodological challenge when the subgroups are defined by post-treatment, rather than pre-treatment, variables because if analyses are performed in the same way as with pre-treatment variables, causal interpretations are no longer valid. The authors illustrate a new approach to this challenge within the context of the Infant Health and Development Program, a multisite randomized study that provided at-risk children with intensive, center-based child care. This strategy is used to examine the differential causal effects of access to high-quality child care for children who would otherwise have participated in one of three child care options: no non-maternal care, home-based non-maternal care, and center-based care. Results of this study indicate that children participating in the first two types of care would have gained the most from high-quality center-based care and, moreover, would have more consistently retained the bulk of these positive benefits over time. These results may have implications for policy, particularly with regard to the debate about the potential implications of providing universal child care. © 2002 by the Association for Public Policy Analysis and Management.