Leber hereditary optic neuropathy (LHON)/pediatric onset dystonia is associated with a G to A transition at nucleotide position (np) 14459, within the mitochondrial DNA (mtDNA)-encoded ND6 gene. This mutation has been reported in families presenting with LHON alone, LHON plus dystonia, or pediatric dystonia with typical age of onset less than 5 years. The mutation changes a moderately conserved alanine to a valine at amino acid residue 72, which is within the most evolutionarily conserved region of the ND6 protein. Pediatric onset disease is associated with basal ganglia dysfunction, spasticity, and encephalopathy. We report a family with G14459A mtDNA mutation and a broad spectrum of clinical manifestation. The proband was a 3-year-old girl with anarthria, dystonia, spasticity, and mild encephalopathy. MRI of the brain demonstrated bilateral, symmetric basal ganglia lucencies associated with cerebral and systemic lactic acidosis. Her maternal first cousin presented with a new onset limp and mild hemiparesis along with similar MRI findings with a much milder phenotype. Additional investigation of the family members with the mutation has revealed both asymptomatic and symptomatic individuals with variable clinical and laboratory features of mitochondrial disease. This study re-emphasizes the heterogeneous clinical manifestation of homoplasmic G14459A mtDNA mutation even within the same family, and supports the hypothesis that nuclear genes may play a role in modifying the clinical expression of mitochondrial disease. Published 2003 Wiley-Liss, Inc.