Three hundred and sixty-seven women presenting to the Breast Unit at Guy's Hospital between 1975 and 1990 whose first distant metastasis was in the skeleton were identified and the influence of a number of patient and tumour characteristics on the development and subsequent prognosis of bone metastases was assessed. One hundred and thirty-nine women had disease that remained clinically confined to the skeleton. They were more likely to be older, with lobular carcinoma and to have presented initially with little or no axillary lymph node involvement. The 228 women who subsequently developed disease at extra-osseus sites were more likely to have poorly differentiated ductal tumours and heavy lymph node involvement at primary diagnosis. On multivariate analysis, the clinical and pathological factors of greatest prognostic importance for survival after the development of bone metastases were histological grade (P = < 0.0001), oestrogen receptor status (P = < 0.0001), bone disease at initial presentation (P = < 0.0001), disease-free interval (P = 0.002) and age (P = 0.006).