A case of a 68-year-old man is described who was referred to our department for further treatment of a malignant hemangiopericytoma of the left maxillary sinus. The referral diagnosis was made through an incisional biopsy. After angiographic verification of a highly vascularized tumour, embolisation of the left maxillary artery was performed. Further surgical treatment consisted of a hemimaxillectomy, partial resection of the orbital floor and removal of the ethmoidal cells. The histopathological examination of the removed tumour revealed a cellular infiltrate of spindle-shaped cells in a fibromyxoid stroma containing inflammatory cells consisting of lymphocytes, plasma cells and macrophages. The spindle-shaped cells expressed vimentin and a-smooth muscle actin positive immunophenotypes. Considering the histological features the initial diagnosis of a malignant hemangiopericytoma was withdrawn and the diagnosis of an inflammatory myofibroblastic tumour was made. In view of the changed diagnosis the initially planned exenteration of the left orbit did not have to be performed. This case report shows that an incisional biopsy remains problematic with regard to an accurate diagnosis of spindle-celled soft tissue tumours with overlapping morphological features and different clinical behaviour. In order to avoid unnecessary impairment of quality of life the treatment should not be performed too aggressively until the definitive histological diagnosis has been made on the removed tumour. Keywords: spindle-celled soft tissue tumour, malignant hemangiopericytoma, inflammatory myofibroblastic tumour, maxillary sinus