Out of 180 incubations of human biopsies 33 were from patients with malignant tumors of the oral region. In 9 cases it was possible to analyse the cell cycle of the tumor cells with the double labeling in vitro method (3H and 14C-thymidine). Histologically the tumors were keratinizing squamous cell carcinomas with high labeling indices. The generation time of the tumor cells ranged from 20 to 130 hours. The length of the DNA synthesis phase varied between 7.3 and 17.9 hours. With the knowledge of the cell-kinetic data the patients received an infusion of 5 Fluorouracil throughout the length or parts of the calculated G 1 phase. At the end of the 5 FU infusion there was a constant two-hour pause and after the duration of the calculated DNA synthesis phase the tumors were irradiated with 150 rd per session. The treatment was repeated until a total dose of 6000 rd was achieved. Under the combined therapy of 5 FU and irradiation the inoperable keratinizing highly differentiated squamous cell carcinomas regressed extraordinarily fast. The patients were free of pain after a very short time. No vital tumor tissue was visible in control biopsies. Some patients remained without any tumor recurrence now more than 2 years after onset of therapy. In the discussion it is stressed that the reason for the successful treatment of highly differentiated squamous cell carcinomas with 5 Fluorouracil and subsequent irradiation is not easily explained, because recent experimental findings and impulsecytophotometrical studies on human biopsy material suggest that both the synergistic action of thecytostatic drug and the irradiation as well as an altered recruitment of the tumor cells are responsible for the clinical success rather than a synchronisation effect.