A 44 year-old man was admitted to our hospital in October, 1992 with epistaxis. Fifteen months before admission, thrombocytopenia had been pointed out, but he had no bleeding episode. At admission, neither hepatosplenomegaly nor lymph node swelling was observed. Hematological findings were as follows; WBC 10,000/microliter, Hb 14.0 g/dl, PLT 5,000/microliter, including giant platelets. Platelet associated IgG (PAIgG) was 471 ng/10(7) cells. Bone marrow aspiration revealed normocellularity with moderate increase in the number of mature megakaryocytes, but no dysplastic changes were seen. Biochemical and radiological examinations disclosed neither malignant diseases nor autoimmune diseases. Based on the diagnosis of idiopathic thrombocytopenic purpura, danazol therapy (300 mg/day) was started in November, 1992. After initiation of treatment, the platelet count changed cyclically every 4 weeks. To elecuidate the mechanism of this phenomenon, we periodically evaluated the patient's immunological background. T lymphocyte counts and subset proportions remained almost the same through out the course, whereas NK cell counts and NK activity coincided with the change of the platelet count. These results suggest that in this case NK cells may play a role in the cyclic fluctuation of platelet count by regulating the immune system.