The addition of paclitaxel (Taxol; Bristol-Myers Squibb Company, Princeton, NJ) to the therapeutic armamentarium for breast cancer has resulted in novel opportunities and challenges for the clinician. In a series of trials that began in 1992, the Eastern Cooperative Oncology Group (ECOG) investigated the role of paclitaxel in combination with doxorubicin for the treatment of patients with advanced breast cancer. The design of the first trial, a limited-institution pilot study, involved alternating doxorubicin and paclitaxel as single agents. The alternating schedule was well tolerated and associated with objective responses in seven of 12 patients entered into the trial. In the second trial, a limited-institution phase I trial, bolus doxorubicin was combined with paclitaxel administered over a 24-hour infusion, with administration of the two drugs separated by 4 hours. While no dose-limiting toxicity was seen at doses of 50 and 150 mg/m2 of doxorubicin and paclitaxel, respectively, dose-limiting mucositis occurred at respective doses of 60 and 175 mg/m2. Mucositis was more common when administration of paclitaxel preceded that of doxorubicin than when doxorubicin preceded paclitaxel. Based on these results, the ECOG initiated a phase III trial comparing single-agent paclitaxel, single-agent doxorubicin, and the combination of paclitaxel and doxorubicin. This study, now an Intergroup trial of ECOG, the Southwest Oncology Group, and the North Central Cancer Treatment Group, is designed to accrue 730 patients. This trial will close to accrual in September 1995, and analysis of the trial should provide useful information regarding the potential synergy of doxorubicin and paclitaxel, the degree of cross-resistance between the two compounds, and the relationship between steady-state paclitaxel levels and response to therapy. In addition, ECOG is currently conducting a trial designed to confirm the striking activity of cisplatin and paclitaxel seen in the British Columbia trials. Future trials by the group will examine means of combining paclitaxel with other active agents.