We describe a novel mitochondrial ND2 mutation (T4681C) in a patient presenting with Leigh Syndrome. Biochemical analyses revealed a low isolated complex I activity in patient's fibroblasts, blood and skeletal muscle. Mutant transmitochondrial cybrid clones retained the specific complex I defect, demonstrating the mitochondrial genetic origin of the disease. The mutation leads to a L71P substitution at an evolutionary conserved amino acid stretch. By two-dimensional blue native electrophoresis (2D-BN-SDS-PAGE), decreased complex I levels were observed together with an accumulation of specific assembly intermediates, suggesting that the mutation disturbs the complex I assembly pathway.