Until now it was unknown, whether 5-fluorouracil (5-FU) would be absorbed sufficiently after oral application, so that therapeutical effects could be expected. For this reason a comparative pharmacokinetic study of intravenous versus oral application was performed on six patients, as well as a pilot study on 13 patients with adenocarcinomas of different origins. The results show that 5-FU is absorbed rapidly. The biological availability increases with higher dose, which would indicate a saturation of the "first pass" in the liver. The clinical study shows partial remission in seven patients, with hepatoma and tolerable signs of bone marrow depression, decrease of hemoglobin, leukocytes and platelets after oral application of 5-FU in doses of 1,000-1,250 mg on days 1, 3, 5, 8, 10, and 12. 5-FU can therefore be given successfully at an adequate dose by the oral route.