In phase I and II studies we administered fixed doses of doxorubicin by intravenous bolus 16 hours before escalating doses of paclitaxel (Taxol; Bristol-Myers Squibb Company, Princeton, NJ) for the treatment of patients with advanced breast cancer who had received no prior treatment or who had relapsed after adjuvant therapy. Nineteen patients were entered in the study from April 1994 to February 1995. The median age of participants was 54 years; the median disease-free interval was 328 days. Eleven patients had undergone prior adjuvant chemotherapy, seven had undergone prior hormonal therapy, and six had undergone chest radiotherapy. A total of 128 courses of fixed-dose doxorubicin 50 mg/m2 by intravenous bolus and paclitaxel (doses escalated from 130 to 250 mg/m2, through dose escalations of 30 mg/m2 if maximum tolerated dose was not reached) were repeated every 21 days for a median of seven cycles per patient. Toxicities encountered in this trial included grade 4 neutropenia (20% of courses) and grade 4 thrombocytopenia (3% of courses). No grade 3 or 4 nonhematologic toxicities were observed (World Health Organization grade I peripheral neuropathies and mild myalgias in 37.5% and 30% of courses, respectively). No cardiac toxicity was observed. Responses included six complete responses (31.6%), nine partial responses (47.2%), and three stable disease (15.8%), for an overall response rate of 78.8%. Median duration of overall and complete response was 8+ months and 7+ months, respectively. At dose levels > or = 190 mg/m2, all patients had achieved a response (six complete responses and six partial responses). A phase II trial using fixed doses of doxorubicin (50 mg/m2) and paclitaxel (220 mg/m2) is ongoing. Preliminary data on 71 courses report no cardiac toxicity. Treatment with paclitaxel has been well tolerated at each dose level. The maximum tolerated dose was not reached at 250 mg/m2. No cardiac toxicity was reported. The dosing sequence of doxorubicin followed by paclitaxel is a highly active regimen and needs to be tested in anthracycline patients and in an adjuvant setting.