Twenty-two Jewish patients, belonging to 15 families, 11 of them from Iran and three possibly of Iranian stock, suffered from progressive muscle weakness and wasting. The initial symptom was usually distal leg muscle weakness, appearing in the third or fourth decade and insidiously involving the proximal muscles and to a lesser extent the upper limbs. The quadriceps muscle was consistently spared even in advanced cases. Computerized tomography (CT) scans of muscles demonstrated variable wasting and fatty replacement of limb and axial muscles, while the vastus lateralis muscle retained its normal CT appearance. The typical light microscopy features of the affected muscles were: presence of vacuoles within muscle fibres, internal nuclei, longitudinal fibre splitting and, in severely affected muscles, endomysial fibrosis without, inflammation or fibre necrosis. Electron microscopy suggested that the vacuoles were autophagic. Cytoplasmic and intranuclear inclusions were rare. While electromyography (EMG) revealed presence of spontaneous activity, however, analysis of muscle action potentials, turns-amplitude ratio, macro-EMG and single fibre EMG suggested a primary myopathic disorder. Consanguinity in seven families, the parents being first cousins, and the presence of additional affected siblings of both sexes may suggest an autosomal recessive trait. The presence of this disorder in Iranian Jews may indicate that this is a distinct myopathic entity.