Under Belgian law, offenders deemed to lack criminal responsibility because of insanity receive mandated treatment under the internment law. Population profiles of these forensic patients (‘internees’) are, however, very scarce. In this study, we analysed the demographic, clinical and judicial profile of a large sample of Belgian internees admitted to a secure setting. In addition, differences between internees admitted to a medium versus a high security setting were investigated. Belgian internees were characterised by a large number of personality disorders and a low number of first offenders. Comparative analyses showed substantial differences between the high and medium security settings, with a marked proportion of the forensic patients in high security having committed a sexual offence. Contrary to expectations, more predictors for length of stay were found in the medium security subsample, while admission periods were significantly longer in the high security subsample.