The purpose of this study is to describe gender differences in the use of outpatient mental health services and to identify potential determinants of this use. The study sample, N = 7475 respondents 18-64 years, was drawn from the Mental Health Supplement to the Ontario Health Survey. For theoretical and empirical reasons, type of mental disorder was defined as: a Mood and/or Anxiety Disorder (Mood/Anx) or a Substance Use Disorder and/or Antisocial Behaviours (Subs/Asb) within the past year. Use was defined in relation to providers seen within the past year. Descriptive and multiple logistic regression analyses were employed including type of mental disorder, social and economic factors. Female gender remained positively associated with any use despite adjustments (adjusted OR: 1.7; 95% CI: 1.2: 2.4). The magnitude of this association was consistent across the levels of the study variables and various measures of use except volume of use where there were no gender differences. Mood/Anx appeared to mediate the gender-use relationship and was strongly associated with use (adjusted OR: 8.4; 95% CI: 5.9; 11.9). Subs/Asb was also related to use (adjusted OR: 2.6; 95% CI: 1.5; 4.3) but not to the same degree as Mood/Anx. Mood/Anx explained 60% of the crude Subs/Asb-use relationship. The evidence to suggest that Subs/Asb mediated the gender-use relationship was mixed. These findings raise questions about gender differences in illness and reporting behaviours and the health care system in its preferential treatment of women and those with Mood/Anx.