This study used data from the National Longitudinal Survey of Youth to investigate the effects of participation in Head Start preschool programs on a range of child outcomes. In order to control for selection into the Head Start program, comparisons are drawn between siblings and also between the relative benefits of Head Start versus other preschool programs. The study found large and significant gains associated with attending Head Start, as measured by test scores. These gains were evident relative to children who did not attend preschool, as well as to children who attended non-Head Start preschools. The study also found that while whites and blacks experienced initial gains in test scores as a result of participation in Head Start, the gains of blacks were quickly lost while the gains of whites persisted into adulthood. Head Start participation significantly reduced the probability that a white child would repeat a grade, but had no effect on grade repetition among black children. Relative to children who did not attend preschool, both whites and blacks gained greater access to preventive health services in Head Start and non-Head Start programs. A list of related RAND Corporation papers is included. (Contains 68 references.) (MDM)