BackgroundCommunity organizations can have difficulty implementing evidence-based prevention programs. More research is needed on implementation support interventions designed to help these organizations implement programs with quality.MethodsPreparing to Run Effective Programs (PREP) is a randomized controlled trial testing Getting To Outcomes (GTO), a 2-year implementation support intervention. It compares 15 Boys and Girls Club sites implementing CHOICE (control group), a five-session evidence-based alcohol and drug prevention program, with 14 similar sites implementing CHOICE supported by GTO (intervention group). PREP replicates a previous GTO study that had the same design, but featured a teen pregnancy prevention program instead. All sites received typical CHOICE training. Fourteen intervention sites received GTO manuals, training, and onsite technical assistance to help practitioners complete implementation best practices specified by GTO (i.e., GTO steps). During the first year, technical assistance providers helped the intervention group adopt, plan, and deliver CHOICE. Then, this group was trained on evaluation and quality improvement steps of GTO using feedback reports summarizing their own data, which yielded revised plans for subsequent implementation of CHOICE. This paper presents results regarding GTO’s impact on CHOICE fidelity (adherence, quality of delivery, dosage) and the proximal outcomes of the youth participants (aged 10–14)—attitudes and intentions regarding cigarettes, alcohol, and marijuana use. Fidelity was assessed at all sites by observer ratings and attendance logs. Proximal outcomes were assessed via survey at baseline, 3, and 6 months.ResultsAfter 1 year, fidelity and proximal outcomes were similar between Intervention and control groups. After 2 years (which included GTO quality improvement activities that took place between years 1 and 2), intervention sites had higher ratings of CHOICE adherence and quality of delivery (dosage remained similar). Proximal outcomes did not differ between groups in either year, although there was universally high endorsement of prosocial responses to those outcomes from the start.ConclusionsFindings suggest that systematic implementation support provided by GTO can help community organizations achieve better fidelity. Findings replicate the implementation results from a previous GTO study using the same design, but with a different evidence-based program and different fidelity measures. Although proximal outcomes did not change, in large part due to ceiling effects, the implementation findings suggest GTO can support a variety of programs.Trial registrationThis project is registered at ClinicalTrials.gov with number NCT02135991. The trial was first registered on May 12, 2014.