['To examine the prevalence and profile of people with co-occurring mental and substance use disorders in relation to numerous demographic, diagnostic, and needs-related variables across a comprehensive system of mental health services using a standard methodology.', 'Data were collected on cases (n = 9839) sampled from specialty tertiary inpatient, specialty outpatient, and community-based mental health programs. Status with respect to co-occurring disorders was based on recorded diagnosis of substance use disorder and the substance abuse measure within the Colorado Client Assessment Record. The demographic and needs profile was compared across groups with or without co-occurring disorders within each level of care.', 'Overall, the prevalence of co-occurring disorders was 18.5%, and highest among clients receiving specialty tertiary inpatient care (28%), and within selected subpopulations such as younger adults (55%) and those with personality disorders (34%). There were few differences between groups based on co-occurring disorders in the specialty inpatient programs. For outpatient and community settings, the clients with co-occurring disorders were distinguished by a more impaired and complex needs profile and more likely to be young, single, male, and of low education. Across all levels of care, having a co-occurring disorder was strongly associated with antisocial and challenging behaviour, legal involvement, and risk of suicide or self-harm.'] Do prevalence and profile of people with co-occurring mental and substance use disorders within a comprehensive mental health system?