The research focused on narrow versus broad definitions of mental disorder based on measures of: (1) psychiatric symptoms (the Brief Psychiatric Rating Scale (BPRS); Overall and Gorham, 1962); (2) syndromes (the Diagnostic Profile (DP); Hart and Hemphill, 1989); and (3) disorders (lifetime diagnoses on the Diagnostic Interview Schedule (DIS), Version III-A; Robins and Helzer, 1985). Each participants were administered the BPRS, the DP, and the DIS. The research also analyzed variables related to adjustment problems in the jail, other mental health issues, and any institutional treatment or pharmacological treatment for mental health problems. Results revealed moderate agreement between symptom-based and syndrome-based definitions, especially for the narrow definitions. Furthermore, these definitions yielded similar estimates of prevalence and had similar patterns of associations with variables related to institutional security and mental health problems. The disorder-based definitions, especially the broad ones, had low agreement with the other definitions; yielded higher prevalence rates; and were associated only weakly with institutional security and mental health problems. Findings highlight the need to give more attention to definitional issues in research on mentally disordered offenders. Findings also support the usefulness of definitions based on active psychiatric symptoms associated with major mental illnesses. Tables and 33 references (Author abstract modified)