The usefulness of low dose (600 mg/day) 5'-DFUR for advanced or recurrent breast cancer was evaluated. Eight patients out of 38 showed a partial response (response rate 21.1%), 22 patients had no change, and 8 patients had progressive disease. The response rate was 31.0% in soft tissue and 9.1% in bone, both of which were slight. In both one-shot (200 mg) and continuous (1-4 months) administration of 5'-DFUR (600 mg/day), the concentration of 5'-DFUR of 5-FU in sera reached a plateau from 30 minutes to 2 hours after administration of 5'-DFUR, and disappeared after 3 hours. There were no significant differences in the concentration of 5'-DFUR of 5-FU in sera between one-shot and continuous administration. It is concluded that there was no accumulation of the drug after continuous administration of 5'-DFUR, and that low dose 5'-DFUR is a useful therapy for advanced or recurrent breast cancer.