Little is known about the etiology or pathogenesis of angiosarcoma (AS). We describe a series of 5 cases of AS arising in chronic expanding hematomas. Inclusion criteria were the presence of a hematoma of at least 1-year duration and a thick fibrous wall surrounding the hematoma. Patients were 4 men and 1 woman; ages ranged from 43 to 71 years. Locations were the thigh (3), chest wall (1), and pelvic soft tissue involving the ischial bone (1). Hematoma duration ranged from 2 to 25 years. All cases had large cystic hematomas >10 cm; 2 had prior radiation. Thick fibrous walls surrounded the hematomas, with foci of hemosiderin and foamy histiocytes. Wall thickness ranged from 0.2 to 1.0 cm and varied within lesions. All AS were epithelioid, and in 3 cases the tumor invaded through the cyst wall. Immunoreactive nuclear c-myc was noted in 3/3 cases available for testing. Follow-up disclosed 4 patients developed metastatic disease, 3 of whom died of disease, 4, 8, and 15 months after diagnosis; the fourth patient is alive without disease after chemotherapy at 59 months. One patient without metastases is alive without disease 18 months after diagnosis; this tumor was confined to the cyst without penetration through the wall. We identified 4 similar cases in the literature, 3 as individual case reports (all epithelioid AS), and 1 as part of a series of AS. To our knowledge, this is the first series of AS arising in chronic expanding hematomas. Recognition of this unusual complication should alert clinicians to provide periodic clinical follow-up to these patients and to biopsy any case with sudden or uncontrolled enlargement. We recommend that excised chronic hematomas be well sampled histologically to search for AS and, if identified, to determine its extent and invasiveness.