Vincristine (VCR) and etoposide (VP-16) have been shown to be synergistic in a murine model, and this combination was studied in a phase II trial. Eligibility required measurable disease, a performance status of 0-2, a life expectancy of > or = 2 months, an interval of at least 3 weeks since the receipt of previous radiation therapy or chemotherapy and recovery from related toxicity, no prior treatment with VCR or VP-16, and no more than two prior chemotherapy regimens (only one for treatment of metastatic disease). Treatment consisted of 0.5 mg i.v. (bolus) VCR followed by 200 mg/m2 VP-16 given over 2 h. Both drugs were given daily for 3 consecutive days every 3 weeks (total dose: VCR, 1.5 mg; VP-16, 600 mg/m2). A total of 18 patients with metastatic breast cancer were accured; 14 had adjuvant chemotherapy and 8 had chemotherapy for advanced disease. As judged by International Union Against Cancer (UICC) criteria, one complete response (CR) and three partial responses (PR) were obtained, for a CR + PR rate of 22% (95% confidence interval, 6%-48%). All responders had soft-tissue involvement only. Six patients had stable disease and 8 showed progression. The median time to treatment failure was 3.5 months, and the median survival from study entry was 8.3 months. The major toxicity was myelosuppression, with 9 patients (50%) experiencing a total WBC of < 1,000/mm3. Grade 2-3 neurologic toxicity was noted in 6 patients (33%) and grade 3 nausea and vomiting was noted in 5 (28%). The combination of VCR and VP-16 is active in advanced breast cancer but is not convincingly superior to either of these agents used alone.