There is an unprecedented societal focus on young people’s mental health, including efforts to expand access to child and adolescent mental health services (CAMHS). There has, however, been a lack of research to date to investigate adult mental health outcomes of young people who attend CAMHS.

We linked Finland’s healthcare registries for all individuals born between 1987 and 1992. We investigated mental disorder diagnoses recorded in specialist adult mental health services (AMHS) and both inpatient and outpatient service use by age 29 (December 31, 2016) for former CAMHS patients.

Before the end of their 20s, more than half (52.4%, n = 21,183) of all CAMHS patients had gone on to attend AMHS. The most prevalent recorded adult psychiatric diagnoses received by former CAMHS patients were depressive disorders (30%, n = 11,768), non-phobic anxiety disorders (21%, n = 7,910), alcohol use disorders (9.5%, n = 3,427), personality disorders (9.3%, n = 3,366), and schizophrenia-spectrum disorders (7.6%, n = 2,945). In the total population, more than half of all AMHS appointments (53.1%, k = 714,239/1,345,060) were for former CAMHS patients. More than half of all inpatient psychiatry bed days were for former CAMHS patients (53.1%, k = 1,192,991/2,245,247).

While there is a strong focus on intervening in childhood and adolescence to reduce the burden of mental illness, these findings suggest that young people who receive childhood intervention very frequently continue to require specialist psychiatric interventions in adulthood, including taking up a majority of both outpatient and inpatient service use. These findings highlight the need for a greater focus on research to alter the long-term trajectories of CAMHS patients.