A preoperative adjuvant chemotherapy for the prevention of recurrence of breast cancer is necessary. We administrated UFT orally at 600 mg/day to 87 patients with primary breast cancer and measured the 5-FU level in the tissue and blood. The mean blood level of 5-FU after administration was 0.073 mcg/ml at one hr, 0.081 mcg/ml at 2 hrs, 0.043 mcg/ml at 3 hrs and remained significantly after 5 hrs (0.031 mcg/ml). The mean 5-FU level in the dominant vessels of the breast was also significantly high. The mean tumor 5-FU level was 0.22 mcg/g (0.025-0.768) and normal breast tissue level was 0.048 mcg/g (0.011-0.394). This showed significantly high tumor 5-FU level compared with that of normal (p less than 0.001). In comparison of 5-FU levels with tumor sizes of T1, T2 and T3 and histological patterns (papillotubular, medullary tubular and scinhous) not significant difference was observed. The mean metastatic lymph node level of 5-FU was 0.193 mcg/g (0.070-0.789) and normal lymph node level was 0.093 mcg/g (0.008-0.210) (p less than 0.05). In conclusion, preoperative administration of UFT was considered to be very effective for the primary breast cancer because of high tumor affinity of 5-FU.