PURPOSE: Bone formation in abdominal scar tissue is a form of heterotopic ossification. It is a rare and underreported phenomenon following abdominal surgery. Heterotopic ossification (HO) is the formation of bone where normally no bone is present and two theories on its pathogenesis prevail: (1) physical dislocation of small bony fragments from the xyphoid process or os pubis into the wound, (2) differentiation of locally available multipotent mesenchymal stromal cells into osteoblasts resulting in the calcification of extracellular matrix. Multipotent mesenchymal stromal cells can differentiate into different cell types by exposing them to different stimuli. We hypothesize that pro-osteogenic signals derived from e.g., macrophages steers multipotent mesenchymal stromal cells involved in the wound healing towards osteogenesis. METHODS: In a retrospective case study we analyzed ossified tissue, patient demographics, medical history, number of laparotomies, scar location, indication for surgery and time in which HO occurred. RESULTS: Ten (8 male, 2 female) patients had proven HO. The mean age was 62 (46-80) years. The mean time for HO to occur was 99 (24-382) days after the previous laparotomy. The mean number of relaparotomies was 3 (1-9). CONCLUSION: We conclude that ossification of abdominal scar tissue is a rare but innocent finding and provides interesting leads to other fields of research.