A nerve sheath myxoma involving the breast has been examined by light and electron microscopy, and by immunohistochemical and histochemical methods. Electron microscopically, cells with features indicative of Schwann cells, perineural cells and fibroblasts were identified in the tumour and S-100 protein and vimentin positivity was demonstrated in the tumour cells. The mucoid matrix stained positive for chondroitin-4 or 6-sulphate in keeping with a cartilaginous lesion. These findings are discussed in relation to the uncertain histogenesis and the differential diagnoses of the tumour in the breast.