BackgroundLooked after young people represent a group of people under a supervision order with a local authority. They are frequently reported engaging in poorer health behaviours than are their peers. There are few reported assessments of behaviour change interventions for looked after young people, therefore the effectiveness of existing interventions and insight for the development of further interventions are unknown. Much has been made of the challenge of how to affect key health behaviours in adolescence and hence reduce future inequalities in health. The objectives were therefore to develop and assess a health behaviour change intervention for looked after young people to support healthier lifestyle choices. MethodsThe intervention was developed with a qualitative needs assessment, the general evidence for the effectiveness of interventions around the targeted health issues, and theory and models used in health psychology. The intervention was delivered as a service in NHS Fife, Scotland, by a health psychologist in a personally tailored way to individuals and aimed to motivate and provide looked after young people with the skills for change for sexual health, smoking, activity, healthy eating, alcohol, and drugs. Young people were guided in behaviour change techniques, including action planning to bridge intention-behaviour gaps. Several individual sessions were offered. Assessment of the effect of the intervention was undertaken before and after its delivery with behaviour questions that we developed, on the basis of questions previously used in published work—eg, “How many units do you usually have per week (on average)?” All young people engaged with the service were eligible to take part and were asked for their consent to take part in the assessment. Data were non-parametric and were analysed with the Wilcoxon signed-rank test and McNemar's test. Sessions were further assessed with a behaviour change taxonomy developed by Susan Michie and colleagues. Findings74 young people were referred or self-referred into the service between March 3, 2009, and Aug 31, 2011, and had completed the intervention at the time of the assessment. Because of drop-outs and difficulties gaining consent for some individuals, baseline data were available for 50 young people. Some post-intervention data were available for 37 individuals and analyses undertaken in this group. Despite the small numbers, we recorded positive behaviour change in most areas targeted, including significant findings for the use of pregnancy contraceptives (before: three [21%] of 14 used them; after: ten [71%] used them; p=0·016), attending at a sexual health service (before: three [25%] of 12 had attended; after: nine [75%] had attended; p=0·031), and exercise (median before: 1·7 h/week [IQR 2·5]; median after: 2·9 h/week [2·0]; p=0·028). Drawing on the taxonomy used to audit the sessions, the intervention implemented a wide range of behaviour change techniques including motivational interviewing and action and coping planning. InterpretationThe partnership between Health Psychology and Public Health has allowed the development of an innovative tailored one-to-one service for looked after young people around healthy lifestyle issues. Early assessment results show behaviour change in key risk areas; however, there are several complexities to successful research into or assessment of interventions for vulnerable groups such as looked after young people. A before and after test design without a control group is limited in its power; furthermore, in view of the difficulties engaging young people in assessment, more rigorous research is likely to encounter barriers. There are potentially wider applications for this approach, such as delivery of health behaviour change in school settings and residential care, and for selected vulnerable groups (eg, those in contact with criminal justice). FundingFife Health and Wellbeing Alliance.