Using data from the Canadian Community Health Survey (CCHS) Cycle 1.2, we examine the nature of unmet mental health care needs in Ontario, Canada and how this is associated with socio-demographic, social support, health status and mental health service use factors. Unmet mental health care needs result from experiencing barriers to three issues: acceptability, accessibility and availability. Unmet needs due to acceptability issues are the most frequent type; the largest proportion of people within this category report experiencing unmet needs because they "preferred to manage the problem themselves". Unmet needs are greater among the young and among females. Surprisingly, service users report higher rates of unmet needs than non-users. Some social support variables have associations with unmet needs. Based upon these results, unmet needs pose a major challenge to the health care system since they cannot be resolved solely by enhancing access to and availability of mental health services. Thus, to address unmet mental health care needs, efforts should be focused on the acceptability barriers that women and young people in particular face. Enhancing education and certain social support mechanisms are potential strategies.