Two patients, a father and his 14-year-old son, were suffering from a facioperoneal syndrome, and muscle biopsy findings were consistent with a myotubular myopathy. The father exhibited central nuclei in most muscle fibers, but his son had typical changes exclusively in hypotrophic type I fibers. The cytochemical and ultrastructural analysis revealed a spectrum of pathological changes typical of myotubular myopathy. Energy-dispersive electron probe x-ray microanalysis was performed on 6- to 12-microns thick freeze-dried cryosections visualized in the scanning or scanning transmission mode of electron microscopy. We found a high intracellular sodium and chlorine concentration and a low potassium concentration in comparison with control muscles. These changes pointed in the direction similar to results from human fetal muscle. The changes in the intracellular elemental composition may indicate a membrane pump dysfunction, which might be caused by a partial arrest in muscle fiber maturation.