Aims: The study investigates differences in dual diagnosis disorders among patients in psychiatric and in substance use treatment services. Method: The sample (n = 159) comprised patients from four treatment settings: community mental health service, inpatient psychiatric ward, drug treatment service, alcohol treatment service. Patients who screened positive for dual diagnosis received a detailed multi-axial assessment of substance use and mental health problems. Results: Important clinical differences were found between dual diagnosis patients in psychiatry and substance use treatment services. Dual diagnosis patients in psychiatry services were more likely to have a psychotic illness: those in the inpatient psychiatry service had the most severe mental health problems, and were exposed to the greatest risks to their safety. Psychiatric outpatients were least likely to have an alcohol dependence disorder, although a high rate of drug use disorders (mainly involving cannabis) was found among the psychiatric inpatients. High rates of affective disorders and suicide risk were found among patients in all treatment services. Conclusions: The term ÔdualÕ diagnosis can be misleading. Dual diagnosis patients present with complex mental health, physical health, psychosocial disability, and safety needs, and the pattern of problems presented by dual diagnosis patients varies across different treatment services.