OBJECTIVE: To investigate the relationship between social exclusion and outcomes of mentally ill patients with substance abuse problems receiving assertive outreach treatment in London. METHOD: Analysis was conducted on data on 580 patients from the 'Pan-London Assertive Outreach Study (PLAO)'. Data were collected using clinician-rated scales of alcohol and drug abuse in the last 6 months before baseline. Outcomes - hospitalization and compulsory hospitalization- were assessed over a 9-month follow-up period. RESULTS: The analysis identified a group of patients with substance abuse who suffer from social exclusion and forensic problems (n = 77, 15.8%), and had poorer outcomes than the rest of the patients in terms of hospitalization (52% vs. 36% respectively) and compulsory (39% vs. 22% respectively) hospitalization. CONCLUSION: There is a distinct group of patients whose treatment requires social inclusion and forensic expertise. Future research into which model of service is suitable for these patients and in what setting is recommended.