Cyclic fluctuations of the leukocyte and platelet counts were observed in five patients with chronic myelogenous leukemia during treatment with hydroxyurea. The cyclic periods ranged from 30 to 50 days, were similar in each individual, but the peak-to-trough values of single cycles varied in magnitude. A second biorhythm was a cyclic undulation of the leukocyte count with heights of the peaks at nine-to-20-month intervals. Rhythmic variations of the platelet counts were also observed; the period and phase of the cycle being the same as that of the leukocyte cycle. No cyclic oscillations were noted in the hemoglobin or reticulocyte counts. The recognition of the phenomenon may aid in the use of hydroxyurea or other chemotherapeutic agents in the control of myelogenous leukemia. Chronopharmacology may more accurately develop the use of antitumor agents by its investigation of drug effects on biologic characteristics as a function of biologic timing.