OBJECTIVES: To compare the mental health status of early adult and mid-life Australian women according to sexual orientation. METHODS AND SAMPLE: Cross-sectional analyses of the Australian Longitudinal Study on Women’s Health (ALSWH) surveys for the younger (22-27 years) and mid-age (50-55 years) cohorts. Women were classified into one of four groups: exclusively heterosexual, mainly heterosexual, bisexual and lesbian. Regression analyses were used to examine the effects of sexual orientation on mental health after adjusting for age, region of residence and education and to assess the potential mediating roles of stress, abuse and social support. RESULTS: Younger, mainly heterosexual, bisexual and lesbian women had poorer mental health outcomes than exclusively heterosexual women on all outcome measures except anxiety in lesbian women, even after adjustment for age, region and education. Mid-age mainly heterosexual women had poorer mental health on all outcomes except for medically diagnosed anxiety and bisexual women had significantly higher odds of self-harm than exclusively heterosexual women. All non-heterosexual women in both cohorts reported higher levels of stress and lifetime abuse. Controlling for stress, abuse and social support attenuated the mental health findings. CONCLUSIONS: The poorer mental health in young non-heterosexual women and mid-life mainly heterosexual women highlights the need for health care providers to be particularly sensitive to mental health issues in these women. Stress, social support and lifetime abuse may play a role in explaining the poorer mental health and discrimination may also be important.