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There may be an association between deep-vein thrombosis and the development of cancer, especially in patients who have recurrent episodes of unknown cause. Deep-vein thrombosis is the formation of a blood clot in a major vein. Among 250 patients with a first episode of deep-vein thrombosis who were followed over a two-year period, 145 were diagnosed with idiopathic venous thrombosis, or thrombosis of unknown cause, and 105 developed a venous thrombosis secondary to another disease. Eleven patients (7.6%) with idiopathic venous thrombosis developed symptomatic cancer during the follow-up period, compared with two patients (1.9%) with secondary venous thrombosis. Thirty-five patients with idiopathic venous thrombosis had recurring episodes of thrombosis. Six of these patients (17%) later developed cancer. Some patients with recurrent idiopathic venous thrombosis may be suffering from cancer, but have not developed symptoms yet.