["Journal of Child Psychology and Psychiatry, EarlyView. ", "\n\nBackground\nCredible long‐term outcomes from randomized trials evaluating the effectiveness and cost‐effectiveness of preventive programs for mental health problems are needed. We compared long‐term effects of the Mind My Mind (MMM) transdiagnostic cognitive behavioral therapy (CBT) program to management as usual (MAU).\n\n\nMethods\nThe study was a pragmatic, multisite, randomized superiority trial (2017–2019) involving youths aged 6–16 years with anxiety, depressive symptoms, and/or behavioral disturbances, recruited through family self‐referral. The MMM intervention included 9–13 weekly CBT sessions. The primary outcome was change from baseline in parent‐reported impact of mental health problems at 3‐year follow‐up post‐randomization using the Strengths and Difficulties Questionnaire (SDQ) impact scale. Register‐based outcomes tracked youths' psychiatric diagnoses in mental health services. To assess cost‐effectiveness, we calculated the incremental costs and incremental quality‐adjusted life years (QALYs). All primary analyses followed the intention‐to‐treat (ITT) approach. Clinical trials registration: ID NCT04804917.\n\n\nResults\nAmong 396 youths randomized (baseline mean [SD] age, 10.3 [2.4] years; 52.0% boys; MMM n = 197, MAU n = 199), the 3‐year follow‐up (median 167 weeks; range 124–203 weeks) primary outcome data were available in 69.0% and 59.3%, respectively. The decrease in SDQ‐impact‐score from baseline to 3‐year follow‐up (4.12→1.79 points [MMM] and 4.21→1.85 [MAU]) was similar (between‐group difference, 0.06 [95% CI −0.41 to 0.52]; p = .81). An equal proportion (25%) of youths in MMM and MAU were diagnosed with any mental disorder during follow‐up (HR 1.01, 95% CI 0.68–1.50). Total costs over the intervention period were higher in the MMM group (incremental costs 3,014 Euros [95% CI: 2.174–3.855]). Cost‐effectiveness analyses favored MMM: QALY net gain 0.121 (95% CI 0.045–0.196); the cost‐effectiveness ratio was 24,789 Euro/QALY.\n\n\nConclusions\nAlthough MMM was potentially cost‐effective, the beneficial effects diminished over 3 years post‐treatment. The findings highlight the need for strategies to sustain long‐term effects.\n\n"]