Parents of children with developmental delays consistently report higher levels of child behavior problems and also parenting stress than parents of typically developing children. This study examined how mothers' positive beliefs influence the relation between children's behavior problems and mothers' parenting stress among families of children who are developmentally delayed (DD: n = 72) or typically developing (TD: n = 95) and assessed at ages 3, 5, and 7 years. Positive beliefs had a main effect on parenting stress at all ages, which was mediated by child behavior problems for mothers in the DD group at every age and across time. In the TD group, mediation was found at age 3 years. Additionally, support was found for a moderation effect of positive beliefs on the relation between child behavior problems and parenting stress, but only in the DD group at age 3. These findings have implications for interventions drawing on Seligman's (1991) work on learned optimism, the positive counterpart of learned helplessness. (Contains 1 figure and 5 tables.)