Previous analyses demonstrated an elevated occurrence of perceived unmet need for mental health care among persons with co-occurring mental and substance use disorders in comparison with those with either disorder. This study built on previous work to examine these associations and underlying reasons in more detail. Secondary data analyses were performed on a subset of respondents to the 2002 Canadian Community Health Survey (unweighted N=4,052). Diagnostic algorithms classified respondents by past-year substance dependence and selected mood and anxiety disorders. Logistic regressions examined the associations between diagnoses and unmet need in the previous year, accounting for recent service use and potential predisposing, enabling, and need factors often associated with help seeking. Self-reported reasons underlying unmet need were also tabulated across diagnostic groups. Of persons with a disorder, 22% reported a 12-month unmet need for care. With controls for service use and other potential confounders, the odds of unmet need were significantly elevated among persons with co-occurring disorders (adjusted odds ratio=3.25; 95% confidence interval=1.96-5.37). Most commonly, the underlying reason involved a preference to self-manage symptoms or not getting around to seeking care, with some variation by diagnosis. The findings highlight potential problems for individuals with mental and substance use disorders in accessing services. The elevated occurrence of perceived unmet need appeared to be relatively less affected by contact with the health care system than by generalized distress and problem severity. Issues such as stigma, motivation, and satisfaction with past services may influence help-seeking patterns and perceptions of unmet need and should be examined in future work.