Background Severe obesity in adolescents has a profound impact on current and future health. Metabolic and bariatric surgery (MBS) is increasingly used in adolescents internationally. However, to our knowledge, there are no randomised trials examining the currently most used surgical techniques. Our aim was to evaluate changes in BMI and secondary health and safety outcomes after MBS. Methods The Adolescent Morbid Obesity Surgery 2 (AMOS2) study is a randomised, open-label, multicentre trial done at three university hospitals in Sweden (located in Stockholm, Gothenburg, and Malmo). Adolescents aged 13-16 years with a BMI of at least 35 kg/m2, who had attended treatment for obesity for at least 1 year, passed assessments from a paediatric psychologist and a paediatrician, and had a Tanner pubertal stage of at least 3, were randomly assigned (1:1) to MBS or intensive non-surgical treatment. Exclusion criteria included monogenic or syndromic obesity, major psychiatric illness, and regular self-induced vomiting. Computerised randomisation was stratified for sex and recruitment site. Allocation was concealed for both staff and participants until the end of the inclusion day, and then all participants were unmasked to treatment intervention. One group underwent MBS (primarily gastric bypass), while the other group received intensive non-surgical treatment starting with 8 weeks of low-calorie diet. The primary outcome was 2-year change in BMI, analysed as intention-to-treat. The trial is registered at ClinicalTrials.gov, NCT02378259. Findings 500 people were assessed for eligibility between Aug 27, 2014, and June 7, 2017. 450 participants were excluded (397 did not meet inclusion criteria, 39 declined to participate, and 14 were excluded for various other reasons). Of the 50 remaining participants, 25 (19 females and six males) were randomly assigned to receive MBS and 25 (18 females and seven males) were assigned to intensive non-surgical treatment. Three participants (6%; one in the MBS group and two in the intensive non-surgical treatment group) did not participate in the 2-year follow-up, and in total 47 (94%) participants were assessed for the primary endpoint. Mean age of participants was 15 center dot 8 years (SD 0 center dot 9) and mean BMI at baseline was 42 center dot 6 kg/m2 (SD 5 center dot 2). After 2 years, BMI change was -12 center dot 6 kg/m2 (-35 center dot 9 kg; n=24) among adolescents undergoing MBS (Roux-en-Y gastric bypass [n=23], sleeve gastrectomy [n=2]) and -0 center dot 2 kg/m2 (0 center dot 4 kg; [n=23]) among participants in the intensive non-surgical treatment group (mean difference -12 center dot 4 kg/m2 [95% CI -15 center dot 5 to -9 center dot 3]; p<0 center dot 0001). Five (20%) patients in the intensive non-surgical group crossed over to MBS during the second year. Adverse events (n=4) after MBS were mild but included one cholecystectomy. Regarding safety outcomes, surgical patients had a reduction in bone mineral density, while controls were unchanged after 2 years (z-score change mean difference -0 center dot 9 [95% CI -1 center dot 2 to -0 center dot 6]). There were no significant differences between the groups in vitamin and mineral levels, gastrointestinal symptoms (except less reflux in the surgical group), or in mental health at the 2-year follow-up. Interpretation MBS is an effective and well tolerated treatment for adolescents with severe obesity resulting in substantial weight loss and improvements in several aspects of metabolic health and physical quality of life over 2 years, and should be considered in adolescents with severe obesity. Copyright (c) 2023 Published by Elsevier Ltd. All rights reserved.