Background: Few large sample studies have examined psychiatric morbidity among former child welfare/protection clients. In this study, risks for suicide attempts and severe psychiatric morbidity in younger years were assessed for former child welfare clients in ten national birth cohorts, comparing them with general population peers and inter-country adoptees. Methods: We used national register data for almost one million people: 22,305 former child welfare clients who had experienced interventions before their teens, 955,326 general population cohort peers and 12,240 inter-country adoptees. Multivariate Cox regression models were used to estimate risks of hospitalisation for suicide attempts and psychiatric disorders from age 13 to age 18-27. Results: Former child welfare clients were in year of birth and sex standardised risk ratios (RRs) four to five times more likely than peers in the general population to have been hospitalised for suicide attempts. They were five to eight times more likely to have been hospitalised for serious psychiatric disorders in their teens, four to six times in young adulthood. High excess risks were also found for psychoses and depression. Individuals who had been in long-term foster care tended to have the most dismal outcome. Adjusting for birth parents' hospitalisations with a psychiatric diagnosis or for substance abuse, and for birth-home-related socio-economic factors, reduced excess risks to around twofold. Conclusions: Irrespective of issues of causality, findings suggest that former child welfare/protection clients should be considered a high-risk group for suicide attempts and severe psychiatric morbidity. Results have substantial practice implications for mental health and social agencies serving this group in adolescence and/or young adulthood.