OBJECTIVES: In 2008, breast cancer accounted for 16% of cancer deaths in British women. Although early stage breast cancer is treatable, prognosis for metastatic breast cancer (mBC) cancer patients is poor. Information on incidence of HR+ mBC in the UK is not readily available, as data on stage was not collected by cancer registries until 2008. In this study, we estimate the incidence of HR+ mBC in the UK. METHODS: Data reported by the International Agency for Research on Cancer was used to estimate breast cancer incidence for 2008.  Values were cross-checked with Cancer Research UK. We then estimated the number of patients that were likely to be HR+ and the number that were likely to progress to metastatic disease using data from published literature and from the National Institute for Health and Clinical Excellence. Surveillance, Epidemiology, and End Results data were used to further cross-check values. RESULTS: It is estimated that of 46,458 incident breast cancer patients per annum in the UK (89 per 100,000), 34,834 are HR+ (75%).  Based on 17 regional registries in the US, 1761 women were reported to be HR+ with metastatic disease (63.5% of mBC cases) in 2008. Most literature-based values estimate that up to 50% of breast cancer patients progress to metastatic disease. Based on this assumption, among UK HR+ cases, an estimated 17,421 will likely progress to metastatic disease (50% of HR+ cases or 37.5% of incident breast cancer cases).   CONCLUSIONS: There is a large gap in the literature on the incidence and prevalence rates of HR+ mBC in the UK. Our estimates revealed that most newly diagnosed breast cancer in the UK is HR+, and about half will progress to metastatic disease.  The large number of HR+ mBC patients in the UK represents a significant clinical and health care burden.