Subjects were administered the National Institute of Mental Health Diagnostic Interview Schedule. Jail prevalence rates were then compared to estimate severe mental disorder among mail jail detainees. For all race-age subgroups, observed jail rates of schizophrenia, major depression, and mania were two to three times higher among Cook County detainees than in the general population. Moreover, these prevalence rates likely underestimated the true prevalence of mentally ill persons processed through the criminal justice system. Samples obtained at the jail level omitted all persons who were arrested, but not incarcerated because they were diverted to a mental health facility during their pretrial hearing. The findings do not definitively show whether mental disorder is a casual determinant of a criminal career or merely a frequent trait among offenders. Since disorders such as schizophrenia, depression, and mania require immediate attention, jails must routinely screen incoming detainees for severe mental disorder. In addition, the prevalence of detainees who are actively psychotic suggests that jail administrators must negotiate program relationships with mental health facilities. In accordance with the American Bar Association's mental health criminal justice standards, mentally ill detainees who have committed minor crimes should be diverted to the mental health system. An appendix summarizes recent research on the prevalence of mentally disordered persons in jails. 65 references and 3 tables