Fifty-nine patients with ovarian carcinoma were studied with particular reference to abnormalities of the coagulation mechanism. The notable findings were the high incidence of episodic thrombocytosis (73%) and the relation of these peaks of thrombocytosis to periods of expected maximum tumour break-down. The incidence of pulmonary embolism for the series was 44%, with the majority occurring in the group demonstrating significant thrombocytosis; in this latter group, pulmonary embolism constituted the major cause of death. The abnormalities in coagulation are discussed, together with pragmatic significance and methods of control.