Single-agent paclitaxel (TAXOL) was administered to 79 patients with stage IV breast cancer. Twenty-eight patients had no prior chemotherapy (for metastatic disease), and 51 patients had extensive exposure to other chemotherapeutic agents before beginning the 24-hour paclitaxel infusion. Routine use of recombinant human granulocyte colony-stimulating factor helped to ameliorate neutropenia, the dose-limiting toxicity, in some cases. Other toxicity was generally mild to moderate. Paclitaxel was more active in patients whose stage IV disease had not yet been exposed to chemotherapy, but activity was seen in the patients previously treated extensively as well. There is a strong clinical suggestion of non-cross-resistance with doxorubicin. In one case, an excellent response in previously irradiated skin was seen. Paclitaxel is a very promising agent for the treatment of metastatic breast cancer.