Background. Some mental disorders have been associated with increased likelihood of sexual offending in adolescents (and adults), but relevant studies tend to be of established sex offenders. Aims. To examine relationships between adolescent mental disorders and subsequent involvement in sex offending and to explore any predictive value of primary diagnoses for subsequent interpersonal offending, whether sexual or violent. Methods. We analyse national register-based longitudinal data on males in Finland admitted for their first psychiatric inpatient treatment between the ages of 13–17 in the period 1980–2010 (N = 6749). Cox regression was used for the analysis of multivariate associations. Results. A subsequent criminal record for sex crime in the 10-year follow up was rare among former child and adolescent psychiatric (CAP) inpatient males (1.5%). Having a subsequent criminal record for non-sex-related violent crime was more common (25%). Time to either sex crimes or non-sex-related violent crimes after a first CAP inpatient treatment was 3–4 years. Whilst the risk of committing non-sex-related violent crimes was elevated in all diagnostic groups compared to those with schizophrenia spectrum disorders, the risk of committing subsequent sex crimes was elevated only in the group with substance use, conduct or personality disorders. Among those with pre-existing criminal history of sex crime, the risk of a subsequent criminal record for sex crime after CAP treatment was increased 11-fold, but the risk for later non-sex-related violent crimes was not increased compared to the rest of the male adolescent CAP population. Conclusions and implications for practice. In this first longitudinal study of criminal convictions for sex offending after a period of inpatient psychiatric treatment as an adolescent such convictions were rare, but the difference in post discharge risk of further convictions for sexual offending and non-sexual violent offending raises questions about whether more attention and specific treatment for aberrant sexual behaviours is needed for male adolescents with severe mental disorders.