Background: Some hereditary palmoplantar keratodermas (PPK) have been defined at the molecular level. Objective: Our purpose was to establish the cause of a hereditary PPK with unique histopathologic findings in the epidermis. Methods: Investigative studies included light and electron microscopy and determination of genomic DNA sequence. Results: Six patients with PPK were found to have unique changes in the epidermis characterized by orthokeratosis, parakeratosis, perinuclear vacuolization, and keratohyalin granules that varied in size and shape and were located in the cell periphery. Electron microscopy showed the perinuclear region contained many ribosomes and vacuoles and was surrounded by a tonofibril shell. Family involvement suggested a dominant disorder. However, no mutation of keratin genes 1, 6a, 9, or 16 was found. Conclusion: The histopathologic features of this unique PPK most closely resemble Curth-Macklin ichthyosis for which the genetic basis has not been established. Further genetic studies are needed. (J Am Acad Dermatol 1997; 39:29–33.)