Two siblings presented with encephalopathy, lactic acidosis, and hypocitrullinemia. Muscle and liver biopsies were considered for respiratory chain studies, but because of hypocitrullinemia, molecular analysis for maternally inherited Leigh syndrome was first performed, revealing in both siblings the mitochondrial DNA T8993G mutation (95% heteroplasmy), allowing to avoid tissue biopsies. Hypocitrullinemia, an occasional finding in mitochondrial diseases, has been specifically associated with T8993G mutation. However, only few patients have been reported, and the prevalence of hypocitrullinemia in 8993 mitochondrial DNA mutations is unknown. In a small series of 16 Leigh syndrome patients, sensitivity and specificity of hypocitrullinemia (< or = 12 micromol/L) for 8993 mitochondrial DNA mutations were 66% and 85%, respectively. Although studies in larger cohorts are necessary, we suggest considering T8993G mutation early in the diagnostic evaluation of infantile mitochondrial diseases with hypocitrullinemia, which minimizes the need for invasive procedures associated with a small but nonnegligible risk of complications and incorrect diagnosis.