BACKGROUND: Cell physiology is regulated along the 24-h time scale by a circadian timing system composed of molecular clocks within each cell and a central coordination system in the brain. The mammalian molecular clock is made of interconnected molecular loops involving at least 12 circadian genes. The cellular clocks are coordinated by the suprachiasmatic nuclei, a hypothalamic pacemaker which also helps the organism adjust to environmental cycles. The rest-activity rhythm is a reliable marker of the circadian system function in both rodents and man. This circadian organization is responsible for predictable changes in the tolerability and efficacy of anticancer agents, and possibly also in tumor promotion or growth. METHODS: Expected least toxic times of chemotherapy were extrapolated from experimental models to human subjects with reference to the rest-activity cycle. The clinical relevance of the chronotherapy principle, i.e. treatment administration as a function of rhythms, has been demonstrated in randomized multicenter trials. RESULTS: Chronotherapeutic schedules have been used to safely document the activity of the association of oxaliplatin, 5-FU and leucovorin against metastatic colorectal cancer and to set up a new medicosurgical management of this disease which achieved unprecedented long term survival. CONCLUSION: The chronotherapy concept offers further promises for improving current cancer treatment options as well as for optimizing the development of new anticancer or supportive agents.