OBJECTIVES: To determine impacts on social-emotional development at school entry of a pediatric primary care intervention (Video Interaction Project [VIP]) promoting positive parenting through reading aloud and play, delivered in 2 phases: infant through toddler (VIP birth to 3 years [VIP 0-3]) and preschool-age (VIP 3 to 5 years [VIP 3-5]). METHODS: Factorial randomized controlled trial with postpartum enrollment and random assignment to VIP 0-3, control 0 to 3 years, and a third group without school entry follow-up (Building Blocks) and 3-year second random assignment of VIP 0-3 and control 0 to 3 years to VIP 3-5 or control 3 to 5 years. In the VIP, a bilingual facilitator video recorded the parent and child reading and/or playing using provided learning materials and reviewed videos to reinforce positive interactions. Social-emotional development at 4.5 years was assessed by parent-report Behavior Assessment System for Children, Second Edition (Social Skills, Attention Problems, Hyperactivity, Aggression, Externalizing Problems). RESULTS: VIP 0-3 and VIP 3-5 were independently associated with improved 4.5-year Behavior Assessment System for Children, Second Edition T-scores, with effect sizes (Cohen’s d) ~-0.25 to -0.30. Receipt of combined VIP 0-3 and VIP 3-5 was associated with d = -0.63 reduction in Hyperactivity (P = .001). VIP 0-3 resulted in reduced “Clinically Significant ” Hyperactivity (relative risk reduction for overall sample: 69.2%; P = .03; relative risk reduction for increased psychosocial risk: 100%; P = .006). Multilevel models revealed significant VIP 0-3 linear effects and age ? VIP 3-5 interactions. CONCLUSIONS: Phase VIP 0-3 resulted in sustained impacts on behavior problems 1.5 years after program completion. VIP 3-5 had additional, independent impacts. With our findings, we support the use of pediatric primary care to promote reading aloud and play from birth to 5 years, and the potential for such programs to enhance social-emotional development. © Copyright 2018 by the American Academy of Pediatrics.