Based on preclinical data, we designed a phase I/II clinical trial to determine the efficacy and toxicity of doxorubicin followed by paclitaxel in the treatment of advanced breast cancer (either untreated or relapsed after adjuvant therapy). In the phase I dose-finding study, 19 enrolled patients received bolus doxorubicin (50 mg/m2) and, after a 16-hour interval, a three-hour infusion of paclitaxel in escalating doses from 130 to 250 mg/m2, increased by 30-mg/m2 increments for each dose-level group. The first dose level group (paclitaxel 130 mg/m2) included three patients. The other dose level groups included four patients. Treatment was repeated every three weeks for a maximum of eight cycles. The paclitaxel dose was escalated to 250 mg/m2 without reaching the maximum tolerated dose. In the 128 cycles assessable for toxicity, there were no relevant clinical signs or symptoms of cardiotoxicity. This absence of significant cardiotoxicity required confirmation in a phase II trial. Since a maximum tolerated dose of paclitaxel had not been reached during the first study and an increasing risk of neutropenia and peripheral neurotoxicity was feared if doses continued to escalate, a phase II confirmatory study was begun to evaluate treatment with fixed doses of doxorubicin (50 mg/m2) and paclitaxel (220 mg/m2), using the same schedule and interval as in phase I. The 13 patients enrolled in phase II received a total of 95 cycles of therapy; in 10 cycles (three patients) dose reductions were necessary because of toxicity. However, no significant clinical cardiotoxicity was observed in 12 of the 13 patients. One patient experienced an asymptomatic, transient decrease in left ventricular ejection fraction at a cumulative doxorubicin dose of 400 mg/m2. Overall clinical responses included 10 complete remissions (31.3%) and 15 partial remissions (46.9%) for an objective response rate of 78.1%. At 16 months' median follow-up, the median time to progression for all patients is nine months. The high response rate obtained in the phase I/II studies and, in particular, the absence of significant cardiotoxicity require confirmation in further clinical trials. To date, two confirmatory phase II trials are ongoing in our institutions.