The confluence of the deinstitutionalization of people with mental illness and the massive incarceration of drug offenders has dramatically raised the number of substance abusers with mental illness in American jails. This study sought to identify correlates of comorbidity and service gaps among jail inmates using data from the 2002 Survey of Inmates in Local Jails. Results suggest that offenders with comorbidity can be satisfactorily identified according to their past exposure to certain risk factors. One third of the jail population met the comorbidity criteria and 64% of them did not receive any treatment or counseling service while incarcerated. Policy implications are discussed.