Background People with a major mental disorder are at increased risk of committing crimes, especially violent crimes, compared with the general population. Sub-groups have been identified based on age of onset of anti-social or violent behaviour. Mentally disordered offenders with early onset anti-social behaviour tend to have a lifelong pattern of it, but in a clinical setting, are they easily identifiable as a distinct sub-group?Aims Our main aim was to establish whether distinct groups of early and later onset offenders can be identified from the standard clinical record of men with schizophrenia spectrum disorders selected for hospital treatment after conviction for a serious crime, and to test the hypothesis that even in such a clinically selected group, early onset offending would be associated with subsequent persistent and versatile offending.Methods A retrospective case file review of all 83 men with a schizophrenia spectrum disorder in a complete 2-year discharge cohort from one specialist secure forensic hospital unit (2001-2002).Results A sub-group of patients with early onset anti-social behaviour was confirmed. Prior to this specialist hospitalisation, this group had sustained significantly more criminal convictions and were more criminally versatile than their late onset peers.Conclusions The recognition of clinically meaningful sub-groups among hospitalised offender patients seems straightforward, and may be useful in the development of more specifically focused intervention and in making sense of more longer-term outcomes. Copyright (c) 2010 John Wiley & Sons, Ltd.