Background: Most evidence about what works in transitional care comes from small studies in single clinical specialties. We tested the hypothesis that exposures to nine recommended features of transitional healthcare were associated with better outcomes for young people with long-term conditions during transition from child-centred to adult-oriented health services. Methods: This is a longitudinal, observational cohort study in UK secondary care including 374 young people, aged 14-18.9years at recruitment, with type 1 diabetes (n = 150), cerebral palsy (n = 106) or autism spectrum disorder with an associated mental health problem (n = 118). All were pre-transfer and without significant learning disability. We approached all young people attending five paediatric diabetes centres, all young people with autism spectrum disorder attending four mental health centres, and randomly selected young people from two population-based cerebral palsy registers. Participants received four home research visits, 1 year apart and 274 participants (73%) completed follow-up. Outcome measures were Warwick Edinburgh Mental Wellbeing Scale, Mind the Gap Scale (satisfaction with services), Rotterdam Transition Profile (Participation) and Autonomy in Appointments. Results: Exposure to recommended features was 61% for 'coordinated team', 53% for 'age-banded clinic', 48% for 'holistic life-skills training', 42% for 'promotion of health self-efficacy', 40% for 'meeting the adult team before transfer', 34% for 'appropriate parent involvement' and less than 30% for 'written transition plan', 'key worker' and 'transition manager for clinical team'. Three features were strongly associated with improved outcomes. (1) 'Appropriate parent involvement', example association with Wellbeing (b=4.5, 95% CI 2.0-7.0, p=0.001); (2) 'Promotion of health self-efficacy', example association with Satisfaction with Services (b=-0.5, 95% CI -0.9 to -0.2, p=0.006); (3) 'Meeting the adult team before transfer', example associations with Participation (arranging services and aids) (odds ratio 5.2, 95% CI 2.1-12.8, p<0.001) and with Autonomy in Appointments (average 1.7 points higher, 95% CI 0.8-2.6, p<0.001). There was slightly less recruitment of participants from areas with greater socioeconomic deprivation, though not with respect to family composition. Conclusions: Three features of transitional care were associated with improved outcomes. Results are likely to be generalisable because participants had three very different conditions, attending services at many UK sites. Results are relevant for clinicians as well as for commissioners and managers of health services. The challenge of introducing these three features across child and adult healthcare services, and the effects of doing so, should be assessed.