Objectives: To estimate the incidence of metastatic breast cancer (MBC) in Australian women with an initial diagnosis of non-metastatic breast cancer. Design, setting and participants: A population-based cohort study of all women with non-metastatic breast cancer registered on the New South Wales Central Cancer Register (CCR) in 2001 and 2002 who received care in a NSW hospital. Main outcome measures: 5-year cumulative incidence of MBC; prognostic factors for MBC. Results: MBC was recorded within 5 years in 218 of 4137 women with localised node-negative disease (5-year cumulative incidence, 5.3%; 95% CI, 4.6%- 6.0%); and 455 of 2507 women with regional disease (5-year cumulative incidence, 18.1%; 95% CI, 16.7%-19.7%). The hazard rate for developing MBC was highest in the second year after the initial diagnosis of breast cancer. Determinants of increased risk of MBC were regional disease at diagnosis, age less than 50 years and living in an area of lower socio-economic status. Conclusions: Our Australian population-based estimates are valuable when communicating average MBC risks to patients and planning clinical services and trials. Women with node-negative disease have a low risk of developing MBC, consistent with outcomes of adjuvant clinical trials. Regional disease at diagnosis remains an important prognostic factor.