A patient with neurofibromatosis developed neurofibrosarcoma (NFS). The tumour was composed of spindle-shaped cells with atypical nuclei arranged in a fascicular or sheet-like fashion. The tumour cells had discontinuous basement membrane-like structures and were positively stained with the monoclonal antibody against S100 protein alpha chain. Both the primary tumour and its cultured cells contained GM3 and GD3 as major gangliosides. The ganglioside pattern of the primary tumour corresponds to that of NFS in our previous study. In the cultured NFS cells, the relative amount ratio of GM3 to GD3 is almost reversed compared to that of the primary tumour. This reversion may reflect some environmental influence on the ganglioside metabolism of NFS.