OBJECTIVE: This article presents the prevalence, patterns, and sequences of severe psychiatric disorders and substance use disorders among female jail detainees. METHOD: Subjects were a randomly selected, stratified sample of 1,272 female arrestees awaiting trial at the Cook County Department of Corrections in Chicago. Independent clinical research interviewers administered the National Institute of Mental Health Diagnostic Interview Schedule Version III-R to assess comorbid psychiatric disorders and substance use disorders. RESULTS: Eight percent of the women had both a current severe mental disorder and a current substance use disorder. Nearly three-quarters of those with severe mental disorders also met criteria for one or more substance use disorders. CONCLUSIONS: Because most detainees return to their communities in a few days, these findings have implications for treatment of high-risk women throughout the mental health system.