This study investigated the effects of retention or promotion in 1st grade on growth trajectories in mathematics and reading achievement over the elementary school years (Grades 1-5). From a large multiethnic sample (n = 784) of children who were below the median in literacy at school entrance, 363 children who were either promoted (n = 251) or retained (n = 112) in 1st grade could be successfully matched on 72 background variables. Achievement was measured annually using Woodcock-Johnson W scores; scores of retained children were shifted back 1 year to permit same-grade comparisons. Using longitudinal growth curve analysis, trajectories of math and reading scores for promoted and retained children were compared. Retained children received a 1-year boost in achievement; this boost fully dissipated by the end of elementary school. The pattern of subsequent retention in Grades 2, 3, and 4 and placement in special education of the sample during the elementary school years are also described and their effects are explored. Policy implications for interventions for low-achieving children are considered. (Contains 8 tables, 4 figures and 5 footnotes.)