IntroductionYouth involved in the justice system have high rates of alcohol and other drug use, but limited treatment engagement. JJ-TRIALS tested implementation activities with community supervision (CS) and behavioral health (BH) agencies to improve screening, identification of substance use service need, referral, and treatment initiation and engagement, guided by the BH Services Cascade and EPIS frameworks. This paper summarizes intervention impacts on referrals to treatment among youth on CS. MethodsThis multisite cluster-randomized trial involved 18 matched pairs of sites in 36 counties in seven states randomly assigned to core or enhanced conditions after implementing the core intervention at all sites for six months. Enhanced sites received external facilitation for local change team activities to reduce unmet treatment needs; Core sites were encouraged to form interagency workgroups. The dependent variable was percentage referred to treatment among youth in need (N = 14,012). Two-level Bayesian regression assessed factors predicting referral across all sites and time periods. Generalized linear mixed models using logit transformation tested two hypotheses: (H1) referrals will increase from baseline to the experimental period, (H2) referral increases will be larger in enhanced sites than in core sites. ResultsAlthough the intervention significantly increased referral, condition did not significantly predict referral across all time periods. Youth who tested drug positive, had an alcohol/other drug–related or felony charge, were placed in secure detention or assigned more intensive supervision, or who were White were more likely to be referred. H1 (p < .05) and H2 (p < .0001) were both significant in the hypothesized direction. Interaction analyses comparing site pair differences showed that findings were not consistent across sites. ConclusionsThe percentage of youth referred to treatment increased compared with baseline overall, and enhanced sites showed larger increases in referrals over time. However, variations in effects suggest that site-level differences were important. Researchers should carry out mixed methods studies to further understand reasons for the inconsistent findings within randomized site pairs, and how to further improve treatment referrals across CS and BH systems. Findings also highlight that even when CS agencies work collaboratively with BH providers to improve referrals, most justice-involved youth who need SU services are not referred.