Oxidative phosphorylation deficiencies can be caused by mutations in either the nuclear genome or the mitochondrial genome (mtDNA); however, most pathogenic mutations reported in adults occur in mtDNA. Such mutations often impair mitochondrial translation, and are associated with a characteristic muscle pathology consisting of a mosaic pattern of normal fibres interspersed with fibres that show mitochondrial proliferation (ragged-red fibres) and little or no complex IV (COX) activity. We investigated two adult patients with a severe mitochondrial myopathy in whom all muscle fibres showed mitochondrial proliferation with barely detectable COX activity - a pattern never before reported. Biochemical studies of the respiratory chain in muscle showed decreased activities of complexes I and IV (5% of control) and complex II+III (41% of control). Immunoblot analysis of nuclear and mitochondrial subunits of complexes I, III and IV showed a greater than 90% decrease in the steady-state level of these subunits in mature muscle, but no change in nuclear-encoded subunits of complexes II and V. A generalized mitochondrial translation defect was identified in pulse-label experiments in myotubes, but not in myoblasts cultured from both patients. This defect moved with the nucleus in patient cybrid cells. Myoblasts from one patient transplanted into the muscle bed of SCID mice differentiated into mature human muscle fibres that displayed a defect similar to that seen in the patient muscle. These results suggest a defect in a developmentally regulated nuclear factor important for mitochondrial translation in skeletal muscle.