Prospective controlled randomized study. Aim of this study is to assess the feasibility and sensitivity of CT scanning of the thorax, abdomen and pelvis for the detection of occult cancer in patients with idiopathic venous thromboembolism. Patients presenting with acute idiopathic venous thromboembolism, free from already known cancer and in whom a routine battery screening has excluded the presence of cancer, are randomized to receive either a CT scanning of the thorax, abdomen and pelvis (completed by mammography if not already performed in the past year, and by gastroscopy and/or colonoscopy in patients with positive hemoccult) or a diagnostic programme freely decided by attending physicians. Patients of either group in whom the search for cancer is negative are followed-up for two years to register the development of clinically symptomatic malignant disease. The rate of cancer detection and that of cancer development are compared between the two study groups.