An otherwise healthy 55-year-old white woman had acute onset of right arm swelling. No precipitating factors were identified. Venograms revealed an occluded subclavian vein, and catheter-directed thrombolysis was performed. After lysis a persistent residual intraluminal filling defect was identified, with persistent symptoms. Partial claviculectomy was performed, the mass was removed, and patch venoplasty carried out, with good outcome. Pathologic analysis demonstrated the mass to be a Masson's hemangioma, a papillary proliferation of thin-walled capillaries intimately associated with thrombus. Considered a benign intravascular lesion, the treatment of choice is complete excision.