Eighteen patients with advanced breast cancer were entered into a phase II study of fludarabine phosphate. Fludarabine phosphate was given by continuous infusion for 5 days, at a starting dose of 20 mg/m2 per day for patients previously treated with two or more regimens and 25 mg/m2 per day for minimally treated patients with less than two prior regimens; therapy was repeated every 3-4 weeks. Of the 18 patients, 11 had undergone more than two prior regimens and 7 patients had undergone one prior regimen. One patient achieved a partial response (PR) for 22 months. Myelosuppression was the most common toxicity observed. Four patients developed mild nausea and vomiting and two developed a nonspecific dermatitis that resolved spontaneously. No renal, hepato-, or neurotoxicity was observed. Our study demonstrates that in heavily pretreated patients, fludarabine phosphate given on this schedule has minimal efficacy in treating advanced breast cancer. This drug might possibly have shown more activity in a previously nontreated patient population. However, patients with advanced breast cancer, who have not undergone previous treatment are not often encountered.