Telangiectatic osteosarcoma, being a subtype of osteosarcoma, is rare. In this paper, one case is reported. The patient, a sixteen year old man, was admitted into our hospital in 1983 because of swelling and pain in the left knee for 3 months. On physical examination, a mass, 7 x 5 x 5 cm in size, was found in the distal end of the left thigh. A misdiagnosis of hemangioma of the bone was made by puncture biopsy before operation. After three months, the disease progressed and amputation had to be performed. The pathological diagnosis of telangiectatic osteosarcoma was established after operation. Finally, clinical and roentgenographic manifestations, histological and ultrastructural findings, biologic behavior, diagnosis and differential diagnosis are discussed. In particular, the differentiation from aneurysmal bony cyst and hemangioma of the bone is emphasized.