The brittle fingernail is a common complaint, but the features of the cellular structure of the nail plate remain unclear. In this study, clipped nailplates from two persons with severely brittle nails, one female aged 26 years and one male aged 82 years, were observed by light and electron microscopy and compared with normal nail plates. Numerous cracks were observed in clipped brittle nails, but not in normal nails, on light microscopy. When the deep areas of nail plates of the clipped normal nails were observed by electron microscopy, intercellular boundaries appeared intermingled, and two thin, electron-dense layers were observed in a narrow intercellular gap. In contrast, in brittle nails, marked dilatation of intercellular spaces was frequently observed and electron-dense layers were either not seen or were disrupted. When clipped normal nails were dehydrated in a desiccation chamber, similar dilatations - though not so severe -were observed, without evident cracks. These results suggest that dilatation of the intercellular space between nail keratinocytes is correlated with brittle nails and that dehydration may result in such intercellular dilatation.