This is the author accepted manuscript. The final version is available from Springer Verlag via the DOI in this record. ; There are a growing number of school-based interventions designed to promote children’s social and emotional learning. One such intervention, PATHS (Promoting Alternative Thinking Strategies), was evaluated in a randomised controlled trial involving 5074 pupils aged 4–6 years at baseline in 56 primary schools across a large city in the UK. The programme was implemented for two academic years. The primary outcome measure was the teacher-rated Strengths and Difficulties Questionnaire (SDQ). A secondary measure was the PATHS Teacher Rating Scale (PTRS). Observations of child and teacher behaviours were undertaken in a third of intervention and control schools using the Teacher–Pupil Observation Tool (T-POT). Regarding fidelity, dose and adherence were measured via weekly logs completed by teachers, and a semi-structured questionnaire completed by PATHS coaches was used as a global measure of fidelity (capturing adherence, dose and quality). A cost-consequence analysis examined programme costs from a multi-agency public sector perspective. At 1 year post-baseline, there were no statistically significant differences between the programme and control groups on the SDQ subscales or the SDQ total difficulties and impact scores. There were statistically significant differences favouring the programme group for six out of 11 subscales on the secondary outcome measure (PTRS). At 2 years post-baseline, there were no statistically significant differences between the groups on either measure. Fidelity, according to the global measure, was relatively strong, and there was no relationship between fidelity and treatment effects. The average cost of PATHS was £12,666 per school or £139 per child. The study, which was fully powered and independent of the programme developer, shows no statistically significant effect of the programme on child behaviour or emotional well-being. Trial registration site and number: www.controlled-trials.com: ISRCTN 32534848. ; This study was funded by Birmingham City Council (BCC) as part of its “Brighter Futures” strategy for children’s services.