The purpose of the study was to determine the incidence and risk factors for venous thrombosis in patients with a peripherally inserted central catheter (PICC). A retrospective study of all upper extremity venous duplex scans was carried out in the Vascular Medicine department from year 2000 to 2002 inclusive. A chart review of positive scans was undertaken to identify possible thrombotic risk factors. Of 317 upper extremity venous duplex scans carried out, 115, or 32%, were positive for upper extremity deep vein thrombosis. Three main risk factors were identified - presence of a central line, malignancy and administration of chemotherapy. PICC were the most common central line present. Symptomatic thrombosis occurred in 7% of PICC inserted for chemotherapy compared with 1% of PICC inserted for other reasons. Ten per cent of the patients receiving chemotherapy through a PICC developed a thrombosis. The post-thrombotic syndrome was infrequent following upper extremity deep vein thrombosis. Patients receiving chemotherapy through a PICC are at increased risk of thrombosis. There may be a role for prophylactic low-dose anticoagulation in these high-risk patients.