Our goal was to determine if a healing flare response seen on bone scintigraphy occurs following chemotherapy with Taxol (paclitaxel; Bristol-Myers Squibb Co., Princeton, NJ), a novel antimicrotubule agent for metastatic breast cancer. METHODS: We performed 74 bone scans on 21 females with breast cancer and bone metastases entering a Phase II trial of Taxol chemotherapy with granulocyte colony stimulating factor (G-CSF). All patients had baseline scans within 6 wk prior to therapy, after the second cycle (4-6 wk) of Taxol, and then after 6-12 mo. All bone scans were reviewed by two nuclear medicine physicians, without knowledge of the patients' clinical history. Skeletal radiographs, CT and MRI scans, as well as clinical history were compared with scan findings. RESULTS: Seven of the 21 patients showed improvement in bone scan findings. Of these seven, three had a flare response following two cycles (4-6 wk) of Taxol, characterized by increased activity in baseline lesions and the appearance of new lesions, followed by improvement on follow-up scans. Evidence of clinical response (> or = 50% reduction in tumor mass) was seen in all of these patients. Seven patients showed no change in baseline findings on follow-up bone scans. Seven patients had post-Taxol scans showing new lesions, with no overall improvement on later follow-up. CONCLUSION: Flare on bone scintigraphy may be seen shortly after commencing Taxol chemotherapy. Bone scans done within the first 3 mo must be interpreted with caution and should be correlated with clinical and radiological findings to avoid inappropriate discontinuation of Taxol chemotherapy.