Desmoid tumours are rare lesions with a local invasive potential and a risk of recurrence, considered to be benign due to the absence of metastases. They are classified as fibromatoses and may be associated with Gardner's syndrome. The authors report an unusual case, in a 33-year-old woman, of a desmoid tumour invading the right ureter with upper tract dilatation. Etiologic factors (traumatic, hormonal, auto-immune...) are discussed, together with the treatment of choice to lower the risk of recurrence.