Fibromatosis also known as desmoid tumor is an uncommon cause of a mediastinal mass in patients of all ages. Imaging appearance of fibromatosis is generally nonspecific and demands special attention to subtle details to be correctly identified as a possibility. Management of the patient is often complicated by failure to obtain precise pre-operative diagnosis. Location of a mass in the anterior mediastinum with encasement of vital structures is not favourable for complete cure. Although histologically benign, biological behaviour of the lesion varies between benign fibrous proliferation and low-grade fibrosarcoma. We present imaging appearances, surgical management dilemma, and the histopathological details of a case of fibromatosis in the anterior mediastinum in a child.