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