The serial change of various cytokines in the serum from a patient with cyclic thrombocytopenia is described. Interleukin 7, stem cell factor, and transforming growth factor beta 1 synchronized with the platelet count, and there was a significant positive correlation between the three cytokines and the platelet count. Levels of macrophage colony-stimulating factor, thrombopoietin, platelet-associated IgG and erythropoietin changed reciprocally with the platelet count, and there was a significant negative correlation between the platelet count and these cytokines except erythropoietin. No cyclic change was observed in IL-3, IL-6, IL-11, granulocyte-macrophage colony-stimulating factor, or leukaemia inhibitory factor. These observations suggest that this disease involves two cyclic changes: megakaryocytopoiesis and platelet destruction, in both of which the cytokines play an important role.