Many patients with venous thromboembolism are being treated with low molecular weight heparin for extended periods of time. It is not certain if it is necessary to assess anti-Xa levels for extended treatment periods. This study is a prospective assessment of anti-Xa levels in patients on long-term therapy for acute venous thromboembolism who have active cancer. Consecutive consenting patients from one center in a multicenter trial that compared 6 months of low molecular weight heparin with oral anticoagulant therapy were treated with therapeutic doses of dalteparin (200 IU per kilogram) subcutaneously daily. Anti-Xa levels were assessed at the end of weeks 1 and 4, 4–6 hours after injection of dalteparin. Patients were followed for bleeding and recurrent venous thromboembolism. There were 24 patients who had anti-Xa levels measured at weeks 1 and 4. Two other patients had week 1 measurements performed but died before the week 4 sample was collected due to their underlying cancer. The mean anti-Xa levels at weeks 1 and 4 were 1.11 and 1.03 anti-Xa units/ml respectively (P=0.13). These results suggest that for patients with active cancer receiving extended duration therapy with low molecular weight heparin (dalteparin) there is no accumulation of anti-Xa effect over the first month of therapy. Monitoring of anti-Xa levels in this situation is usually not required.