Extreme thrombocytosis associated with malignancy

A case is described in which a platelet count of 6,000,000/mm3 occurred in association with adenocarcinoma of the lung and remitted following definitive radiotherapy of the primary lesion. Despite the magnitude of the platelet elevation, thrombohemorrhagic phenomena were not observed. Extreme thrombocytosis of this magnitude is rare in either primary or secondary disorders of thrombopoiesis. The occurrence of unexplained thrombocytosis demands the exclusion of malignancy.