A case is presented of a multifocal malignant neoplasm involving the skin of the upper back in a 10-year-old boy following radiation therapy to the head and neck for a cerebellar medulloblastoma. Histologically, the neoplastic cells were remarkable for the presence of abundant periodic acid-Schiff (PAS)-positive diastase-resistant intracytoplasmic eosinophilic granules. Immunoperoxidase procedures revealed strong positive staining of the tumor cells with desmin, vimentin, and smooth muscle myosin antibodies, and negative staining for myoglobin, S-100 protein and keratin, thus supporting a smooth muscle line of differentiation for this neoplasm. Electronmicroscopy demonstrated numerous intracytoplasmic autophagic vacuoles that corresponded to the granules observed under the light microscope. Leiomyosarcoma should be entertained in the differential diagnosis of poorly differentiated cutaneous neoplasms histologically characterized by a proliferation of cells containing abundant granular eosinophilic cytoplasm.