The electron microscopic findings in the onychomatricoma are described. In the proximal zone of the onychomatricoma, basal cells have various aspects, some being lacunar while others have only a limited cytoplasmic rim containing mitochondria and tonofilaments. In the parakeratotic cell columns the cells elongate and homogenized tonofilaments appear. Around the lacunae the cells are poorly differentiated and their cytoplasm is granular. It can be concluded that in an onychomatricoma the basal cells have a decreased amount of tonofilaments and desmosomes and that their evolution is not uniform. The tumour can be considered as being the result of a disturbed differentiation of nail matrix cells.