Two patients with massive, tumor-simulating extramedullary hematopoiesis are presented. In 1 patient with homozygous beta-thalassemia, a progressive paraparesis developed because of a mass of ectopic marrow in the spinal epidural space. Surgical removal and radiotherapy were successful. The second patient, who died during surgical biopsy, had posterior mediastinal masses without any apparent cause. A correct preoperative diagnosis may avoid unnecessary surgical intervention. It is suggested that heterotopic marrow should be included in the differential diagnosis of a mass in the lower thoracic region.