Research findings suggest that there may be some academic benefits for those children whose kindergarten enrollment is delayed, and the risk of underachievement seems to be greater for children who are younger when they first enter kindergarten. Although kindergarten enrollment occurs naturally, certain child, family, and childcare factors will likely influence parents' decisions concerning when to enroll their children in kindergarten. Age-of-entry studies have often neglected assignment bias results from those preenrollment factors. In addition, prior research has defined children's relative age outside of the immediate environment, as opposed to conceptualizing relative age within the context where children actually learn. Purpose: This study examined the relationship between early, on-time, or delayed kindergarten enrollment and children's mathematics and reading achievement from kindergarten through third grade. We predicted that the degree to which delayed, on-time, or early enrollment influences children?s reading and mathematics achievement depends on those preenrollment factors that potentially create assignment biases and the relative age of each child to his or her classmates. (author abstract)