A periodic fall of platelet number characterizes an acquired pathological condition named cyclic thrombocytopenia. We observed a patient in whom the episodes of thrombocytopenia (platelet number less than 50 x 10(9)/l) were followed regularly by thrombocytosis (700-2300 x 10(9) platelets/l). The period of platelet count fluctuation was about 40 d. Morphological examination of bone marrow showed the cyclic disappearance of mature and immature megakaryocytes; bone marrow cultures revealed a periodic severe defect of both multilineage and single-lineage progenitor cell growth. When platelet count was falling, a mild defect of platelet aggregation and ATP release was observed, while platelet function was normal when platelet count was rising. Prednisone, thymopentin, high-dose intravenous gamma-globulin and splenectomy were without effect. After 4 years of cyclic platelet and megakaryocyte fluctuations, stable amegakaryocytic thrombocytopenia developed and the patient died of haemorrhagic stroke.