An infant with phenotypic harlequin ichthyosis survived for nine months, then died a crib death. At autopsy, an enlarged, but structurally normal, thymus was found. Light microscopically, the epidermis showed massive hyperkeratosis and variable parakeratosis, and a stain for neutral fat was positive in the upper epidermis and stratum corneum. Electron microscopic study disclosed crystals resembling cholesterol and masses of autophagic vacuoles, many of them glutted with lipid, deposited within cells of the stratum corneum. Biochemically, cholesterol and triglyceride levels in the stratum corneum were sharply elevated (19.8 and 32.0 mg/g of dry weight, respectively). A defect in epidermal lipid metabolism is postulated.