OBJECTIVE The goal of this study was to determine what effects pediatric primary care interventions, focused on promotion of positive parenting through reading aloud and play, have on the socioemotional development of toddlers from low-income, primarily immigrant households. METHODS This randomized controlled trial included random assignment to 1 of 2 interventions (Video Interaction Project [VIP] or Building Blocks [BB]) or to a control group. Mother-newborn dyads were enrolled postpartum in an urban public hospital. In VIP, dyads met with an interventionist on days of well-child visits; the interventionist facilitated interactions in play and shared reading through provision of learning materials and review of videotaped parent-child interactions. In BB, parents were mailed parenting pamphlets and learning materials. This article analyzes socioemotional outcomes from 14 to 36 months for children in VIP and BB versus control. RESULTS A total of 463 dyads (69%) contributed data. Children in VIP scored higher than control on imitation/play and attention, and lower on separation distress, hyperactivity, and externalizing problems, with effect sizes ∼0.25 SD for the sample as a whole and ∼0.50 SD for families with additional psychosocial risks . Children in BB made greater gains in imitation/play compared with control. CONCLUSIONS These findings support the efficacy of VIP, a preventive intervention targeting parent-child interactions, for enhancing socioemotional outcomes in low-income toddlers. Given the low cost and potential for scalability of primary care interventions, findings support expansion of pediatric-based parenting programs such as VIP for the primary prevention of socioemotional problems before school entry.