A case of desseminated alveolar rhabdomyosarcoma in an 18-year-old male with leuco-erythroblastic anaemia is described. Numerous bizarre malignant cells, including frequent multinucleated giant cells, were seen in bone marrow aspirates, and osteolytic lesions appeared late in the clinical course. The primary site of the neoplasm remained undertermined during life and also at necropsy, which revealed minute pulmonary metastases and extensive lymph nodal, pleural and skeletal deposits. The diagnosis was confirmed on necropsy tissue by ultrastructural examination which demonstrated numerous thin (5 nm) and thick (15 nm) intracytoplasmic filaments in tumour cells, sometimes organized in bundles; scattered dense Z-band-like bodies, and rod-shaped structures were also seen. The fine structure of the rhabdomyosarcoma in the present case is compared with previous ultrastructural studies. Elongated, thick intracytoplasmic filaments whose diameter corresponds to that of myosin myofilaments are strong evidence for rhabdomyoblastic differentiation and are considered to be the sine qua non of a positive electron microscopic diagnosis of rhabdomyosarcoma. Orgaized bundles of filaments and Z-band-like dense bodies are usually present, and rod-shaped structures are found infrequently, but none of these are necessary for the ultrastructural diagnosis.