Because of severe and protracted shortages of pediatric behavioral health (BH) specialists, collaboration between pediatric primary care practitioners (PCPs) and BH specialists has the potential to increase access to BH services by expanding the BH workforce. In a previous study, we demonstrated that phase 1 of a behavioral health integration program (BHIP) enrolling 13 independently owned, community-based pediatric practices was associated with increased access to BH services while averting substantial cost increases and achieving high provider self-efficacy and professional satisfaction. The current study was undertaken to assess whether the initial access findings were replicated over 4 subsequent implementation phases and to explore the practicality of broad dissemination of the BHIP model. After phase 1, BHIP was extended over 4 subsequent phases in a stepped-wedge design to 46 additional pediatric practices, for a total cohort of 59 practices (354 PCPs serving >300 000 patients). Program components comprised BH education and consultation and support for integrated practice transformation; these components facilitated on-site BH services by an interprofessional BH team. Outcomes were assessed quarterly, preprogram and postprogram launch. Across combined phases 1 to 5, BHIP was associated with increased primary care access to BH services (screening, psychotherapy, PCP BH visits, psychotropic prescribing) and performed well across 7 standard implementation outcome domains (acceptability, appropriateness, feasibility, fidelity, adoption, penetration, and sustainability). Emergency BH visits and attention-deficit/hyperactivity disorder prescribing were unchanged. These findings provide further support for the potential of integrated care to increase access to BH services in pediatric primary care.