Cancer chronotherapy is attracting attention as a novel and logical therapy in which anti-cancer drugs are administered with optimal timing according to circadian rhythms of anti-cancer action and those of adverse effects on normal cells. Advances in chronobiology have identified the suprachiasmatic nucleus (SCN) as the center of biological rhythms and the area in which clock genes such as PER1, PER2, PER3, CLOCK, BMAL1, TIM, CRY1, CRY2, tau act to generate and coordinate biological rhythms. These findings have led to the development of chronotherapy. Clinically, patients with advanced gastrointestinal cancer have been treated by chronomodulated chemotherapy with good response. For colorectal cancer patients with unresectable liver metastases, chronotherapy with l-OHP + 5-FU + FA (folinic acid) has been reported to allow complete surgical resection of liver metastases, resulting in 39-50% 5-year survival. Many believe that chronotherapy will become accepted as a refined and advantageous therapeutic option for not only cancer but also for other diseases, due to its universally applicable principles.