Patients with the autosomal-dominant form of multiple familial trichoepithelioma develop numerous tumours on the face, neck and upper trunk, beginning in childhood. Malignant transformation of such lesions is quite rare; only one case of "malignant trichoepithelioma" has been reported previously, inferring pilomatrix carcinoma on a histological observation. We report here the case of a patient who developed a malignant neoplasm in a long-standing trichoepithelioma lesion on her buttock. Histopathology revealed a transformation zone between the trichoepithelioma and a malignant tumour mass. This case also showed several features of a malignant neoplasm of trichoblastic origin.