Research has shown that bisexuals have poorer health outcomes than heterosexuals, gays, or lesbians, particularly with regard to mental health and substance use. However, research on bisexuals is often hampered by issues in defining bisexuality, small sample sizes, and by the failure to address age differences between bisexuals and other groups or age gradients in mental health. The Risk & Resilience Survey of Bisexual Mental Health collected data on 405 bisexuals from Ontario, Canada, using respondent-driven sampling, a network-based sampling method for hidden populations. The weighted prevalence of severe depression (PHQ-9 ≥ 20) was 4.7%, possible anxiety disorder (OASIS ≥ 8) was 30.9%, possible post-traumatic stress disorder (PCL-C ≥ 50) was 10.8%, and past year suicide attempt was 1.9%. With respect to substance use, the weighted prevalence of problem drinking (AUDIT ≥ 5) was 31.2%, and the weighted prevalence of illicit polydrug use was 30.5%. Daily smoking was low in this sample, with a weighted prevalence of 7.9%. Youth (aged 16-24) reported significantly higher weighted mean scores on depression and post-traumatic stress disorder, and higher rates of past year suicidal ideation (29.7% vs. 15.2%) compared with those aged 25 and older. The burden of mental health and substance use among bisexuals in Ontario is high relative to population-based studies of other sexual orientation groups. Bisexual youth appear to be at risk for poor mental health. Additional research is needed to understand if and how minority stress explains this burden.