In an attempt to increase the clinical activity of 5-fluorouracil (5-FU) by blocking the thymidine salvage pathway, 15 patients with refractory metastatic breast cancer (MBC) were treated with oral dipyridamole (D): 75 mg p.o. q.i.d. and 5-FU: 400 mg/m2 i.v. by bolus for 5 consecutive days, every 28 days. All the patients were pretreated with 5-FU and an anthracycline-based regimen. Toxicity was minimal, with 8 patients experiencing a D-related moderate headache. Although no objective responses were seen, two patients with 5-FU refractory disease showed tumor shrinkage. A different D schedule and the addition of folinic acid (FA) to 5-FU might provide better results and deserves further evaluation.