In two new cases of centronuclear myopathy, histochemical findings included failure to differentiate fiber type with oxidative enzymes, the presence of core glycogenosis, and core acid phosphatase activity. Fluorescence microscopy demonstrated autofluorescent lipochrome in fiber centers and nonspecific fiber injury in some immunofluorescent preparations. Electron microscopical findings included the observation of unusually small myofibrils arrayed between central nuclei.