The study, a two-arm, randomized controlled, parallel group, superiority trial, aimed to evaluate the implementation and effectiveness of a 12-month one-to-one volunteer mentoring program designed to improve behavioral and emotional outcomes in children aged 5 to 11 years who have teacher- and parent/carer-reported behavioral difficulties. Participants were 246 children (123 intervention, 123 control; mean age 8.4 years; 87% boys) in five sites in London, UK, scoring in the "abnormal" range on the teacher-rated Strengths and Difficulties Questionnaire (SDQ) Total Difficulties measure and in the "borderline" or abnormal range on the parent-rated SDQ Total Difficulties measure. Randomization on a 1:1 ratio took place using a computer-generated sequence and stratifying by site. Data collectors and statisticians were blind to participant allocation status. Outcome measures focused on parent- and teacher-rated child behavior and emotions, and child-rated self-perception and hope. Intention-to-treat analysis on all 246 randomized participants (using imputed data where necessary) showed that at post-intervention (16 months after randomization), there were no statistically significant effects on the primary outcome-parent-rated SDQ Total Difficulties (adjusted standardized mean difference = - 0.12; 95% CI: -0.38 to 0.13; p = 0.33)-or any secondary outcomes. Results from complier average causal effect (CACE) analysis using the primary outcome indicated the intervention was not effective for children who received the recommended duration of mentoring. Exploratory analyses found no sub-group effects on the primary outcome. The article concludes that the mentoring program had no effect on children's behavior or emotional well-being, and that program content needs revising to satisfactorily address key risk and protective factors.