Heterotopic ossification is rarely seen after midline abdominal surgery. The etiology of heterotopic ossification is unknown. Although it is well recognized that heterotopic ossification may contain osteogenic cells and/or hematopoietic cells, to our knowledge, no case has ever been reported to have histologic evidence of hematopoiesis. We report the occurrence of heterotopic bone with bone marrow showing normal trilineage hematopoiesis in the incision scar of a woman who underwent gastric reduction surgery for the treatment of obesity. The literature regarding heterotopic ossification and extramedullary hematopoiesis is reviewed in this report, and discussion focuses on the mechanism of this pathophysiologic process.