This study was undertaken to assess a potential relationship between idiopathic deep venous thrombosis (DVT) and occult cancer. Prospective study with a 2-year follow-up. The Angiology Unit of the First Department of Surgery, University of Athens, Greece, a tertiary referral centre. Two hundred and ninety-three patients with a first episode of venographically or Doppler-proved DVT were included in the study, of whom 264 were followed up for 2 years. After an initial extensive diagnostic workup, including routine blood counts and chemistries, erythrocyte sedimentation rate, CEA levels, chest X-ray and abdominopelvic CT scan, all patients were closely followed up and periodically examined. The incidence of cancer amongst patients with idiopathic and secondary DVT, and the validity of our screening programme. Cancer was diagnosed in 21 out of 84 patients with idiopathic DVT (25%) as compared with eight out of 202 patients with secondary DVT (4%). In 22 out of the 29 cases, cancer was detected during the initial admission, and the remaining seven cases were detected during follow-up. Cancer was diagnosed in 15 asymptomatic, healthy individuals, but only in seven of them was the diagnosis made by CT scan. Occult cancer is fairly common in patients with idiopathic DVT, but the routine use of extensive diagnostic studies for its detection remains to be validated by further prospective studies.