Electron microscopic study was performed on two patients with transient acantholytic dermatosis. Although the lesions in both patients resembled Hailey-Hailey's disease by light microscopy, their ultramicroscopic changes varied in significant details from this disease, and from the other primary acantholytic diseases. In both cases the disorder of fine structure appeared to be related to structural alterations of the periphery of the cell. The damage to the tonofibrils seemed to start proximal to their insertion into the desmosomes and appeared to strip the tonofibrillar material from the desmosome. Concurrently, but probably independently, tonofilamentous material accumulated deeper in the cytoplasm and led to the formation of the mildly dyskeratotic squamous cells found by light microscopy.