OBJECTIVE: To examine associations between poor mental health and sociodemographic, psychosocial and health related variables in midlife Australian women. METHOD: The random population-based sample comprised 13,961 Australian women aged 45-50 years who participated in the baseline postal survey for the Australian Longitudinal Study on Women's Health, conducted in 1996. The outcome measure, poor mental health status, was measured by the Mental Health Index (MH) of the SF-36. RESULTS: Sociodemographic factors associated with poor mental health were low educational levels, being unemployed or engaged solely in home duties, and non-English speaking background (European). Health related factors independently associated with poor mental health were having five or more visits to the doctor in the past year, menopausal status (surgical and peri-menopausal), less exercise, and smoking 20 or more cigarettes per day. Among psychosocial variables, low satisfaction with partner or close relationships, low perceived social support outside family, and more life-events over the past 12 months were independently associated. CONCLUSION: The findings suggest a number of areas in which preventive interventions could be strengthened to improve quality of life among mid-aged women. These include policy changes to promote and support more education and employment opportunities for women before they reach midlife, increase understanding of the link between health risk behaviours and psychological well-being, and provide counselling services to improve women's relational and psychosocial circumstances.