We have analyzed published platelet time series in four normal individuals and 29 putative cyclical thrombocytopenia patients for evidence of significant (p ≤ 0.05) cyclicity. Three of the normal individuals (all males) showed significant cycling (periods T of 23, 30 and 31 days) as did 15 of the cyclical thrombocytopenia patients (T from 13 to 65 days). Cyclical thrombocytopenia seems to be approximately equally prevalent in women and men, though in cases where a definite etiology was suggested an autoimmune origin occurs more frequently in women. We also briefly review the current state of knowledge of the control of platelet production by thrombopoietin (c-Mpl ligand) and previous attempts that have been made to construct mathematical models for the platelet control system.