The concentrations of FT, 5-FU and uracil in lung cancer tissue, healthy lung tissue and serum were measured in 10 patients with primary lung cancer to whom UFT (600 mg) had been administered for one week. No remarkable difference was found in the concentration of FT between both tissues. The concentration of 5-FU in lung cancer tissue was 0.099 +/- 0.051 microgram/g and significantly higher than the level of 0.009 +/- 0.003 microgram/ml in serum (p less than 0.001) and 0.039 +/- 0.021 microgram/g in normal lung tissue (p less than 0.01). The ratio of 5-FU concentration in cancerous tissue to that in the serum was 11.0, while the ratio of the level in cancerous tissue to that in normal lung tissue was 2.5. The concentration of uracil was much higher in cancerous tissue than in normal lung tissue and serum. Maintenance therapy with UFT proved possible for a long period in 11 primary lung cancer patients with negligible digestive side effects.