Objective to re-examine measures to prevent oral mucositis caused by drugs in head and neck cancer patients during cancer treatment by measuring salivary excretion of 5-fluorouracil. Saliva, blood, and urine were simultaneously collected from oral cancer patients and breast cancer patient at the point in time of before, during, and after the administration of 5-FU, then the 5-FU levels of the samples were quantitatively analysed using LC-MS/MS. In all patients, the 5-FU levels in saliva and serum peaked at 30 min to 3 h after the start of 5-FU treatment, and high levels were maintained throughout the administration of the drug. With regard to urinary 5-FU levels, they remained high from 3 to 120 h after the start of 5-FU treatment. After the completion of 5-FU treatment, even though it not appeared in the patients' serum and urine promptly, 5-FU was detected in saliva at 12 h after the completion of 5-FU treatment in one oral cancer patient and at 48 h after the completion of 5-FU treatment in the breast cancer patient. It was suggested that the level of hydration after the completion of chemotherapy may be involved in the differences in 5-FU excretion.