['This study compared baseline characteristics and clinical improvement after 12 months among homeless persons with a diagnosis of serious mental illness with and without a comorbid substance use disorder.', "The study subjects were 5,432 homeless persons with mental illness who were participating in the Center for Mental Health Services' Access to Community Care and Effective Services and Supports (ACCESS) program. Analysis of covariance was used to compare clients who had dual diagnoses and those who did not and to identify any association between service use and clinical improvement.", 'Follow-up data were available for 4,415 clients (81 percent). At baseline, clients with dual diagnoses were worse off than those without dual diagnoses on most clinical and social adjustment measures. Clients with dual diagnoses also had poorer outcomes at follow-up on 15 (62 percent) of 24 outcome measures. However, among clients with dual diagnoses, those who reported extensive participation in substance abuse treatment showed clinical improvement comparable to or better than that of clients without dual diagnoses. On measures of alcohol problems, clients with dual diagnoses who had a high rate of participation in self-help groups had outcomes superior to those of other clients with dual diagnoses. Clients with dual diagnoses who received high levels of professional services also had superior outcomes in terms of social support and involvement in the criminal justice system.'] Do outcomes and service use among homeless persons with serious mental illness and substance abuse?