The present study investigated how changes in specific dimensions of the parent–adolescent relationship predict adolescent engagement in sexual intercourse and oral sex. Longitudinal data from 1,364 participants in the National Institute of Child Health and Human Development (NICHD) Study of Early Child Care and Youth Development were gathered at six time points spanning from first grade to age 15. Latent growth curve modeling was used to investigate whether changes in mother–adolescent and father–adolescent closeness or conflict predicted adolescents’ sexual behavior at age 15. Results indicate that more rapid increase in mother–adolescent conflict predicted engagement in sexual intercourse but not oral sex by age 15, whereas change in mother–adolescent closeness did not predict sexual behaviors. A more rapid decline in father–adolescent closeness and a more rapid increase in father–adolescent conflict predicted more engagement in sexual behaviors by age 15.