Persons with co-occurring mental and substance use disorders are known to have high needs. However, no study has simultaneously calculated the 'fit' between need and use of services for this population using a standardized methodology across a comprehensive community mental health system. The aim of this study was to compare persons with and without co-occurring disorders in terms of their current and recommended levels of care and their need for, and use of, specific mental health, psychosocial and rehabilitative supports within a provincial community mental health system. Trained clinical staff across 407 programs completed 5051 assessments representing 41,051 individuals. Assessments consisted of the Colorado Client Assessment Record and a Support and Service Profile from which co-occurring disorder status and global indices of current and recommended levels of care were derived. Using these data, the mismatch between current and recommended levels of care, and for each of 19 categories of support, were calculated. Without exception, persons with co-occurring disorders, and in particular, those with more severe substance use problems, demonstrated significantly more need and had greater unmet need across a range of service categories. In terms of overall level of care, more than twice as many of the individuals with co-occurring disorders were receiving care at two or more levels below recommended compared to persons without co-occurring disorders. Findings highlight the need for integrated mental health and addictions services to address the needs of persons with co-occurring disorders and reinforce the idea that certain subgroups are particularly vulnerable.