Many physiological variables in hemato-oncology, e.g. the function of liver and kidney and hematopoiesis, show circadian changes. Consequently, the metabolism, elimination and myelotoxicity of cytostatic drugs are subjected to circadian variations in animal and man. Moreover, preliminary clinical data suggest that an optimal circadian timing may even increase the efficacy of anticancer chemotherapy. The recent development of programmable infusion pumps provided a reliable tool for the application of cytostatic drugs at any time of day. As a consequence, controlled clinical trials on the temporal optimization of anticancer chemotherapy seem now to be necessary and feasible.