The mast-cell sarcoma of a bone is described here for the first time. The tumour presented in a 4-year-old boy, with pain, oedema and deformation of his right lower leg. Radiological findings revealed a destructive tumourous mass. Histopathological examination showed the tumour to be composed of large, atypical cells, with hyperchromatic oval and polygonal nuclei. The cytoplasm around them was eosinophilic with many basophilic and toluidine-blue-positive granules. These atypical mast cells were positive for chloroacetate esterase, c-kit, tryptase and negative for myeloperoxidase. The primary disease quickly progressed to mast-cell leukaemia, and despite intensive chemotherapy the patient died 18 months after first symptoms.