Seventy-one patients with breast cancer and bone metastases, together with other assessable sites of disease, were monitored by radiologic skeletal survey, bone scanning, pain charts, bone marrow aspirate, serum calcium, alkaline phosphatase and urine hydroxyproline/creatinine ratio. On the basis of UICC criteria of response in nonosseous sites, 37 were classed as responders and 34 as nonresponders. Responding patients with osteolytic disease frequently showed sclerosis, but only at 6-8 months, whereas patients with mixed lytic/sclerotic or sclerotic metastases frequently showed no change or further sclerosis. Nonresponders most frequently showed progressive lysis. Bone scanning showed clear evidence of improvement or deterioration in 7/21 responders and 8/23 nonresponders who showed no definite evidence of progression or response on skeletal radiography. Pain assessment was also useful in these patients. Neither the bone marrow aspirate nor other biochemical tests were useful in assessing response to therapy. This study concludes that bone scanning and pain assessment are both useful in assessment of response of bone metastases to treatment in some patients and incorporation into a standard criteria of response is recommended.