The existence of malignant mesenchymal tumours with myofibroblastic differentiation (myofibroblastic sarcoma, myofibrosarcoma) is controversially discussed. In the present case report a low-grade myofibroblastic sarcoma of the thoracic wall with varying morphological appearance in numerous recurrences over a number of years is described. The varying immunophenotypes of tumour cells with myofibroblastic differentiation suggests that changing of morphological and immunohistochemical phenotypes may occur in tumour cells with myofibroblastic characteristics corresponding to the "plasticity" of the myofibroblast. The classification of soft tissue tumours is based on a discernible line of differentiation. If cells of a malignant mesenchymal tumour show characteristic features of myofibroblasts by light microscopy and immunohistochemistry, this neoplasm should be classified as a myofibroblastic sarcoma, even if the ultrastructural detection of "fibronexus" is not possible.