Serial bone scans and uptake measurements with99mTc labeled diphosphonate were performed on patients with osseous metastases from primary breast cancer. Eleven of 18 patients showed a transient worsening of the scan appearance shortly after cyclical combination chemotherapy was commenced. The appearance of this “flare” effect could not be correlated significantly with the patients' response to treatment. It is stressed that an apparently deteriorating99mTc diphosphonate bone scan is not an adequate basis on which to terminate a particular therapeutic regime. We have previously reported similar results for a series in which87mSr was the scanning agent.