S-1 has been reported good results in the treatment of stomach cancer. However, a DPD activity change has also been reported as one of the causes for a recurrence though some recurrence cases observing a state of tumor being unchanged continuously for more than 1 year. Therefore the urinary uracil value, which could simply be predicted in the DPD activity at the time of a recurrence after having administered S-1 for a long-term, was measured. And the urinary uracil value was examined whether it could become an index of recurrence compared with a healthy 5-person example as reference. In the result, a monthly difference in change of the urinary uracil values of the reference was small. However, the urinary uracil values in patients who were administered S-1 more than 6 months but recurred, were significantly lower comparing with the healthy 5-person example. We concluded from this study that the change in DPD activity due to an induction of DPD by 5-FU and metastasis of cancer caused the recurrence and lower a urinary value. The urinary uracil value reflecting DPD activity of the whole body could be used as an index of recurrence at the time of long-term dosage of S-1. Furthermore, a measurement of the urinary uracil value seems to be promising for estimation and evaluation of the cure.