This study examined utilization of mental health services and treatment outcomes among 1,091 patients with mental health problems recruited from 39 treatment programs in 13 California counties. We compared three groups: one that received dual treatment from drug and mental health programs (N=294), one that received mental health services only within drug treatment programs (N=578), and one that received no mental health services (N=219). Individuals who received dual treatment had more severe substance use and mental health problems, followed in severity by those who received mental health services within drug treatment, and those who received no services. All groups significantly reduced their psychiatric severity at the follow-up, although there were no group differences in treatment retention and drug use outcomes. These findings suggest that individuals with mental health problems can be successfully treated within drug treatment programs and/or in conjunction with mental health providers.