In order to obtain information about the mechanism of dyskeratosis in Darier's disease, we have examined skin sections at different steps of differentiation at light and electron microscopic level in three patients. Corps ronds occurred at the basal aspect of spinous layer, while grains were abundant in the bulla and in the granular layer of the peripheral epidermis. Corps ronds contained cytoplasmic vacuoles and great number of keratinosomes. Their cytoplasm was divided by a circular ring of tonofilaments into a juxtanuclear and a peripheral compartment. More superficially corps ronds became dyskeratotic keratohyalin-containing cells by acquiring increasing amounts of keratohyalin granules; the cytoplasm turned more vacuolated and poor in keratinosomes. Grains exhibited nuclear pyknosis, shrinkage and densification of mitochondria, numerous tonofilament bundles and confluent vacuoles after disruption of their membrane. In these cells, keratohyalin granules appeared less electrondense and keratinosomes were rare or absent. Horny cells contained empty areas resulting from the persistence of vacuolar spaces. These observations suggest that corps ronds and grains represent stages in the same successive differentiation line and that grains give rise to the changed horny cells occurring in this condition.