The desmoid tumour with infiltrative and destructive growth tendencies, is a rare benign alteration of the connective tissue from musculo-aponeurotic structures and fascial sheaths. The exact pathogenesis is unknown, however trauma (surgery), genetic (Gardener's Syndrome) or hormonal factors (pregnancy) have been implicated. A spontaneous malignant transformation of a desmoid (extra-abdominal fibromatosis) to a fibrosarcoma is a rarity. Only one case is recorded in the literature. We report on the second case in international literature and describe the coherence to an endocrine factor (pregnancy), the high recurrence rate and the difficulty of therapy with regard to the risk of late malignant transformation.