An electron microscope study of tissue diagnosed as white sponge nevus suggested a more advanced process of cellular keratinization in the buccal mucosa than may be appreciated in histologic sections. Ultrastructural features which are normally associated with keratinizing epithelium were observed in the lesion and included (1) a marked increase in the intracellular content of tonofilaments, (2) numerous tonofilament-related keratohyaline granules, (3) membrane-coating granules that possessed an internal striated appearance, (4) the apparent accumulation in the intercellular space of the contents of the striated microgranules, and (5) the presence within some cells of a keratin-like material. Other features included the formation of perinuclear electron-lucent zones and the presence of elongated cytoplasmic processes which produced a complex pattern of interdigitation among the epithelial cells.