The authors prospectively studied 113 consecutive patients with deep venous thrombosis of the lower extremities to determine the most appropriate workup study for searching for a hidden cancer. After a careful physical examination, the following routine tests were performed: erythrocyte sedimentation rate (ESR), whole blood counts, biochemistry, carcinoembryonic antigen (CEA) levels, chest radiograph, upper gastrointestinal endoscopy, abdominal ultrasound and computed tomography (CT) scan. If a malignant lesion was suspected, further appropriate studies were performed. After discharge, periodic follow-up was performed on all patients in the outpatient clinic. A malignant neoplasm was detected in 12 patients. Of these 12 patients, six were asymptomatic with the exception of experiencing thrombophlebitis. Cancer was found more commonly in patients with idiopathic deep vein thrombosis (DVT) (7 of 31 versus 5 of 82 patients with secondary DVT; P = 0.012), and in those patients with abnormal lactic dehydrogenase (LDH) levels (6 of 23 versus 6 of 90; P = 0.007). Abnormal CEA levels allowed diagnosis of two cases of colonic cancer (on colonoscopy). Both ultrasound and CT scan of the abdomen showed two cases of urinary bladder carcinoma at a very early stage. Furthermore, two cases of adenomatous polyps in colon were found, a condition considered by most authors to be a colorectal cancer precursor. In addition, there were five patients with large benign pelvic tumors, and two patients with absent inferior vena cava. The most striking finding was that some cases of cancer were at a very early stage. It was concluded that blood cell counts, LDH, CEA, chest radiograph, and abdominal ultrasonography (or CT scan) should be routinely performed on all patients with deep venous thrombosis (particularly those with idiopathic DVT). Malignancy would not have been recognized in some patients if these tests had not been performed.