Cyclic thrombocytopenia is a rare disorder with cyclic change of the platelet counts. Although the pathogenesis of the disorder has not been clarified, recent reports suggest that periodic destruction and/or ineffective production of platelets may be important causes of the disease. We report a 24-year-old female with the episodes of severe thrombocytopenia (minimum platelet count 2x109/L) followed by normal or higher platelet counts (maximum platelet count 877x109/L). The period of platelet count fluctuation was about 20-40 days. Morphological examination of bone marrow showed the cyclic disappearance of mature and immature megakaryocytes. These findings indicate that the cause of platelet fluctuation is periodic failure of megakaryocytopoiesis.