All patients with carcinoma of the breast seen in this Unit since 1970 were reviewed to study the incidence, prognosis, morbidity and response to treatment of bone metastases. The biological characteristics of the primary tumour were compared in patients relapsing first in bone or liver. Sixty-nine percent of patients dying with breast cancer had bone metastases and bone was the commonest site of first distant relapse. Bone relapse was more common in receptor positive or well differentiated (grade 1) tumours. The median survival was 24 months in those with disease apparently confined to the skeleton compared with 3 months after first relapse in liver. Ten percent of patients with breast cancer developed hypercalcaemia. All had metastatic disease and 85% had widespread skeletal involvement. Fifteen percent of patients with disease confined to the skeleton developed hypercalcaemia. The response in bone to primary endocrine therapy, and chemotherapy, was apparently less than the overall response achieved. A large proportion had apparently static disease reflecting the insensitivity of the UICC assessment criteria. The duration of survival in these patients was similar to responding patients, suggesting a tumour response may occur in the absence of discernable radiological evidence of healing.