The study examined the total mental health agency population of an English county and the total criminal justice population of the same jurisdiction. The purpose was to identify and characterize the mentally disordered individuals who were in contact with the criminal justice system. Data were collected from 1996 to 1998. The amalgamated database consisted of 245,377 subjects. The research population consisted of all persons charged with an offense in the criminal justice system in the county over the 3-year period and all individuals who had a mental disorder (defined as receiving or having received secondary mental health care). All individuals were tracked through the criminal justice system, with information collected on frequency of police contact, type of offenses, and associated problems. This article provides an overview of the proportion of the overall criminal justice population that overlapped with the community mental health trust population as a whole as well as a range of other social and health-care agencies. It then examined the proportion of specific police and probation populations that have had contact with mental health services. It also examined the interaction of criminal justice and mental health variables. The project found that in a county with a population of 800,400, some 30,329 were offenders. More than one-third had received health or social-care services during the 3-year period; 8 percent were mentally disordered. Those offenders ages 25-64 who had police contacts more than once were significantly more likely to be mentally ill. Offenders cited for public order, domestic violence, and substance abuse offenses were more likely to contact the mental health trust. These results can be used as a basis for additional work on target assessment and risk-reduction measures for those most at risk of criminal offending. 6 tables and 13 references