Biopsy specimens of cutaneous lesions from two patients with histiocytosis X of the infantile type (Letterer-Siwe disease) were examined with the electron microscope. More than one half of the dermal macrophages in both specimens contained organelles identical to epidermal Langerhans' cell granules. These organelles appear to arise during the process of endocytosis of extracellular material and are ultimately found in various stages of degradation within Iysosomelike granules. The ultrastructural identity of these dermal macrophages to epidermal Langerhans' cells supports the hypothesis that the epidermal Langerhans' cell is a macrophage.