A subungual solitary glomus tumour was examined by both light and electron microscopy. By light microscopy, typical tumour tissue was seen, surrounded by connective tissue. By electron microscopy, a very close morphological relationship was noted between non-myelinated nerve fibres and mast cells. These mast cells could be divided into two groups: the mast cells of the first group contain numerous mature granules and show few lamellopodia. The distance to the nerve fibre bundles ranged between 2000 and 20 nm. In the second group, the mast cells always showed direct contact with nerve fibers. They had many lamellopodia and contained almost exclusively immature granules. In some cases, invaginations of lamellopodia and broader cytoplasmatic processes into the view that mast cells may play an important part in the function of neurons.