Background. The concept of comorbid mental health problems and substance misuse has gained prominence in the last two decades, due in part to the closure of large psychiatric hospitals and to the increasing prevalence of drug use in the community. This client group has a dual requirement for both medical and social care needs and is at risk for social exclusion. Methods. A retrospective matched case-control study to examine aspects of social exclusion between service users who have comorbid diagnoses and those with a single diagnosis. Samples were drawn from the service users of a mental health Trust in the South-East of England, from both Adult Mental Health (n=400) and Drug and Alcohol services (n=190). Data were collected from Care Programme Approach assessment forms and medical records. McNemar's ? and odds ratios via a conditional logit regression model are used to test for differences in the social exclusion indicators. Results. There were significant differences in social exclusion between the comorbid and singly diagnosed clients of the Adult Mental Health service, but differences were less pronounced between the comorbid and singly diagnosed clients of the specialist Drug and Alcohol service. Conclusions. Recent Government policy advocates treating comorbid clients within mainstream mental health services. Health care workers need to recognise the likelihood of high levels of social exclusion among clients with comorbid problems.