Purpose: The purpose of this article is to present the Gothenland Millennium Cohort, describe the data collection process, present key measures used and summarise some of the key findings to date in order to stimulate collaboration and use of the cohort data. This research programme was originally established to study pathways to alcohol and drug use, behavioural problems, mental health issues, and the factors that promote or prevent these outcomes. The Cohort aims to support scientific research and doctoral education through a longitudinal study that tracks individuals from early adolescence through adulthood. This programme is multidisciplinary (social work, psychology, disability research) with the goal of producing high- quality research that deepens our understanding of how early- life vulnerabilities, risks and protective factors influence long- term wellbeing, including health and welfare, in diverse populations.

Participants: In 2013, all school- registered adolescents, in grades 6 and 7 (aged 12 and 13), in four municipalities in Gothenland region (ie, southern Sweden) born in year 2000 or 2001, were invited to participate. Of 2150 invited adolescents, 1885 (88%) accepted participation in the programme and 1760 (93.4%) participated in at least one of the annual data collection waves up to grade 9 (Wave 4), with participation rates ranging from 70% to 85% per wave. Wave 5 questionnaires were collected during the second year of upper secondary school (grade 11). In Wave 5, half (50.4 %; n = 949) of the adolescents participated. In Wave 6, interviews were conducted with a selection of participants in their last year of upper secondary school (grade 12). Parents were surveyed in Waves 1 and 2 by self- report questionnaires (response rate = 32%; 41%). Data were also gathered from teachers (attrition <30% across Waves 1–4). Child welfare service records and population- based register data have also been collected.

Findings to date: Over 240 publications have been produced as of September 2025 in the areas of disability and everyday functioning, child- parent relationships, child welfare, substance use and criminal behaviour, mental health, trauma, harassment, and sexuality.

Future plans: These include continued investigation of wellbeing and its related indicators during adolescence as well as in emerging adulthood, continued efforts to secure funding and an age 25 expansion of the cohort dat