Paclitaxel (Taxol; Bristol-Myers Squibb Company, Princeton, NJ), the first taxane to enter routine clinical practice, has aroused considerable interest due to its novel mechanism of action and its significant activity in metastatic breast cancer. Given this activity, it seemed logical to attempt to combine paclitaxel with doxorubicin, the other most active single agent in metastatic breast cancer. The Eastern Cooperative Oncology Group performed two trials investigating paclitaxel/doxorubicin combinations in patients with advanced breast cancer in an attempt to identify a tolerable dose and schedule of the combination. In the first trial, paclitaxel and doxorubicin were alternated every 3 weeks in doses of 200 mg/m2 and 75 mg/m2, respectively, for patients who had received no more than one prior chemotherapeutic regimen. Therapy was well tolerated in this setting. At these doses, paclitaxel induced more granulocytopenia and less thrombocytopenia than did doxorubicin. Objective responses (complete and partial responses) were seen in seven of 12 patients; two other patients had improved disease (relief of pain in bony metastases). A second limited-institution Eastern Cooperative Oncology Group trial evaluated paclitaxel and doxorubicin given in combination. In this phase I trial, doxorubicin was given as an intravenous push and paclitaxel as a 24-hour continuous infusion. The sequence of drug administration (D-->P or P-->D) was alternated both between and within patients, so that we might evaluate the effect of administration schedule on toxicity. Therapy was begun at an initial paclitaxel dose of 150 mg/m2 and an initial doxorubicin dose of 50 mg/m2 in six patients, with a subsequent six patients receiving 175 and 60 mg/m2, respectively, of paclitaxel and doxorubicin. In addition, patients received granulocyte colony-stimulating factor 5 micrograms/kg/d. While therapy at the initial dose level was well tolerated, dose-limiting mucositis was seen at the second dose level, although only when paclitaxel preceded doxorubicin. This suggests that sequence of drug administration in paclitaxel-based regimens may play an important role as a determinant of toxicity and (perhaps) efficacy, a finding similar to that seen when paclitaxel and cisplatin were combined in patients with ovarian cancer. Based on this study, we identified the sequence of doxorubicin (50 mg/m2) followed by paclitaxel (150 mg/m2) to be the maximum tolerated dose. This combination is currently being compared with paclitaxel alone and doxorubicin alone in patients with advanced breast cancer in an intergroup trial led by the Eastern Cooperative Oncology Group.