Background In the Netherlands, young offenders who have been convicted of a particularly serious offence may be subjected to a so-called ‘Placement in an Institution for Juveniles’ (PIJ) measure if they are considered to pose a high ongoing risk to public safety. They form a rarely studied distinct group. Treatment in specialist forensic custodial institutions for young people (FYCI) is an intervention of last resort and costly. The most serious young offenders tend to be the hardest to rehabilitate while preventing further offending. Treatment is focussed on reducing risk of harm as well as improving health and other protective factors. Aims To explore the contribution of treatment in an FYCI under a forensic treatment order—the PIJ-measure—to the reduction of risk of reoffending. Methods In a pre–post intervention study, the Juvenile Forensic Profile (JFP) was used to score complete case files of 178 young offenders at the start and end of their placement in an FYCI under the PIJ-measure, 59% of those serving between the years 2013 and 2016 inclusive. The JFP covers risk and protective factors in seven domains encompassing criminal behaviour, family, environment, risk factors, psychopathology, psychology and behaviour during incarceration. Change or stability in scores was tested against reincarceration within 2 years of PIJ-measure completion. Results Impulse control and alcohol and drug use problems showed the greatest improvements. Behaviour that deteriorates during the stay is primarily related to obtaining more autonomy during reintegration efforts, including furlough. Reincarceration in the 2 years of community follow-up was unusual (13.5%). The two main variables associated with reincarceration were problematic behaviour during the pre-discharge year and lack of behavioural improvement during treatment. Conclusions Outcomes of mandatory treatment in this group of serious young offenders have not previously been studied in a rigorous pre–post intervention study design. We found evidence of an overall ...