One unknown factor in the link between crime and mental illness is whether or not mentally ill offenders are more liable than others to arrest. Ninety-one mentally ill, and 76 normal, criminally offending men were asked about the circumstances of their offence and arrest. A majority of mentally ill men had been arrested at the scene of the crime, and more than a quarter of the schizophrenic group had reported themselves to the police. We argue that when many offenders are either unreported or undetected, the increased vulnerability of the mentally ill to detection and arrest makes them disproportionately liable to detection. Attempting to assess the relationship between crime and mental illness is thus extremely difficult.