Results from the Addiction Severity Index (ASI) and the Minnesota Multiphasic Personality Inventory (MMPI) showed that a significant number of inmates manifest symptoms of mental illness. On the basis of an empirically derived criterion measure from the MMPI, inmates were divided into "high psychopathology" (HP) and "low psychopathology" (LP) groups. Evidence of greater psychopathology among inmates in the HP group was corroborated by self-reported symptoms of mental illness from the ASI. HP inmates compared to others sampled were more likely to report lifetime use of amphetamines, hallucinogens, and heroin. They also reported slightly more alcohol and cocaine use in the month prior to treatment. Higher psychopathology was associated with more problems in employment/financial support, family and social relationships, physical health, and within the legal system. The study findings emphasize the need for mental health screening, assessment, and treatment to supplement substance abuse services provided within the criminal justice system.