Two cases of membranous lipodystrophy (T. Nasu) were reported in 24- and 32-year-old men. The roentgenogram showed multiple symmetrical radiolucencies of limb bones where the marrow fat was replaced by numerous characteristic undulating membranes. These membranes consisted of complicated membrane structures, the inner side of which proved to be smooth membrane structures under the electron microscope. Numerous minute tubular structures projected perpendicular to the interstitium from the inner smooth membrane structures. The microscopical findings of subcutaneous fat tissue were described, and the histogenesis of membrane structures under the electron microscope was reported.