Summary. Background: Upper extremity deep vein thrombosis (DVT) can result in fatal pulmonary embolism if not treated. Patients with malignancy may be at particularly high risk. Heparin or low-molecular-weight heparin followed by warfarin has been used as standard treatment for lower extremity DVT. However, a paucity of studies exist reporting the efficacy and safety of these regimens in patients with upper extremity DVT. We studied the effectiveness and safety of treatment with dalteparin sodium followed by warfarin and also dalteparin sodium monotherapy for 3 months in patients with confirmed upper extremity DVT. Methods: Consecutive patients with confirmed upper extremity DVT received daily dalteparin sodium for 5–7 days followed by warfarin therapy for 3 months (phase I) or dalteparin sodium monotherapy for 3 months (phase II). The primary outcome measure was the incidence of new symptomatic venous thromboembolism during the 3-month follow-up period. The outcome measure of safety was the incidence of major and minor bleeding. Results: Of 631 consecutive patients screened, 74 were eligible and 67 enrolled. No patients receiving either phase I (0%; 95% CI, 0–12%) or phase II (0%; 95% CI, 0–9%) therapy had venous thromboembolism on 3-month follow-up. One patient (4%; 95% CI, 0–18%) receiving phase I therapy experienced major bleeding. Five patients died during the follow-up period; none were attributed to pulmonary embolism. Conclusions: Patients with upper extremity DVT may be treated safely with either dalteparin sodium followed by warfarin or dalteparin sodium monotherapy for 3 months with a good prognosis.