This paper reports the findings from Phase I of an ongoing prospective, longitudinal study of mentally ill offenders. The study documents the nature, extent and location of mentally ill offenders in the criminal justice system in Southern Alberta, evaluates mechanisms which are used to identify mentally ill offenders in need of treatment, identifies existing psychiatric services available within the criminal justice system and documents their psychiatric service utilization patterns. A study cohort of 611 was identified at their time of arrest and followed through the justice process from initial detention until final release. Part I on this study focuses on the period of arrest. More specifically, police perceptions of disturbed behaviour are examined. The prevalence of police-identified offenders is reported and police judgments regarding possible causes of disturbed behaviour (that is, mental illness, drugs, alcohol) and recommendations for psychiatric treatment are examined. Discrepancies between official recommendations made by police to a bail magistrate (and recorded on the arrest report) and unofficial recommendations collected as part of the study are explained in terms of differing decision-making models; clinical versus legal. Finally, the importance of police perceptions concerning the presence of mental illness as opposed to substance abuse are identified as important factors which influence the decision to recommend psychiatric examinations.