Two cases of malignant tumours of the nerve sheaths are reported. In the first patient, who had von Recklinghausen’s disease, light- and electron-microscopic investigations showed an undifferentiated mesenchymal tumour of the radial nerve with the property of producing collagen fibres, an aspect characteristic of sarcoma of the fibroblastic type. In the second case a pleomorphic cell sarcoma of the cranial nerves of the left cerebellopontine angle was present. The morphologic aspects of these tumours show them to be primary mesenchymal tumours of the nerve (neurofibrosarcoma, neurogenic sarcoma), originating in the fibroblastic-like elements of the nerve sheaths. The clinical and nosologic framework of this type of malignant tumour of the nerve sheaths is discusse