Mitochondrial DNA mutations cause several human diseases, (eg, Leber's hereditary optic neuropathy). Wolfram syndrome (characterised by diabetes insipidus, diabetes mellitus, optic atrophy, and deafness) also has, in some cases, a mitochondrial origin. The disease, often familial, has been well documented as an autosomal recessive disorder, and most of the clinical phenotypes are consistent with an ATP supply defect that is often seen in mitochondrial-mediated disorders. We propose a dual genome defect model for Wolfram syndrome in which nuclear genetic defects or mitochondrial genetic defects can independently lead to the disease. This model suggests that besides a mitochondrial gene defect alone, a nuclear gene defect, which interferes with the normal function of mitochondria (probably with a normal mitochondrial genome), can also be the underlying explanation for the pleiotropic features of Wolfram syndrome. This hypothesis explains how an autosomal recessive disorder can result in mitochondrial dysfunction, and has a general application in the identification of candidate genes for the various important phenotypes (eg, deafness and diabetes mellitus) seen in mitochondrial disorders.