Severe mental illness and substance abuse are serious problems alone, and when they are combined they can be even more devastating. Treatment for dual diagnosis is best when the treatment of each disorder is integrated into one program. There are various forms of dual diagnosis treatment that were explored. Existing dual diagnosis treatment research will be presented. The purpose of the study is to evaluate an existing dual diagnosis and severe mental illness program that provides integrated mental health and substance abuse treatment. The research design is a pre-experimental one group pretest posttest design (Campbell & Stanley, 1963). Fifty-two people were studied to determine the program effectiveness. The effectiveness of the program was determined by analyzing the data collected on the consumers pretest and posttest measures. The measures used in the research were the BASIS-32, MAI, BHS, BDI, and CAR. Subjects in the study had a mean age of 30. There were 18 females and 34 males. Of the participants, 11.3% had schizophrenia, 13.2% had schizoaffective disorder, 7.5% had bipolar, 11.3% had depression, 5.7% had substance abuse, 43.4% had a dual diagnosis with mental illness and substance abuse, and 5.7% had schizotypal personality disorder. The average length of stay for the consumers was 4 months of treatment. All clients were prescribed medications at discharge with the exception of one client. The results of the study indicate that the BASIS-32 showed statistically significant improvement in results for subjects from pretest to posttest. All other inventories yielded changes that were not statistically significant. Treating dually diagnosed clients in an integrated treatment center is imperative to decrease duplication of services, mixed messages, and exacerbation of problems. In this study, the dual diagnosis program was seen to have some efficacy in treatment. Limitations of the study include a low N, which affects the power of the study to determine statistically significant changes in client inventory scores from pretest to posttest. Other limitations include no comparison group nor any control group. Despite the limitations, the study demonstrates that people improve while in dual diagnosis treatment, even those clients with a single diagnosis.