A low-grade fibromyxoid sarcoma arising in the thigh of a 16-year-old Japanese girl is described. The patient had a well-circumscribed mass measuring 3.5 cm in its greatest diameter within her left vastus medialis muscle and a 6-month history of pain. Microscopically, the tumour was not encapsulated and filtrated into adjacent skeletal muscle. The tumour was characterized by poor to moderate cellularity, a proliferation of bland-appearing spindle tumour cells, and alternating fibrous and myxoid areas with a whorled pattern of the tumour cells. Neither cellular atypia nor mitotic figures were observed. There was no tumour necrosis. Immunohistochemically, the tumour cells were diffusely and strongly positive for vimentin and desmin. Some cells contained alpha smooth muscle actin. They were uniformly negative for CAM5.2, epithelial membrane antigen, muscle-specific actin (HHF35), factor-VIII-related antigen, S-100 protein, neurofilament, CD34, and CD31. The tumour had a diploid DNA content with S-phase fractions of 6.6% by flow cytometry. The patient was alive with no evidence of disease 11 months after excision.