The dynamics of oxygen delivery and utilization are examined in a variety of mitochondrial disorders during rest, exercise and post exercise. We used a non-invasive optical technique to measure the oxygen consumption in the exercising limb in normal subjects and 5 patients with cytochrome c oxidase deficiency. We also examined 6 patients with MELAS and MERRF syndrome. We measured near-infrared spectra of hemoglobin in the gastrocnemius muscle during treadmill exercise. Normal subjects demonstrated a sustained deoxygenation during exercise, indicating an efficient utilization of delivered oxygen. Patients with cytochrome c oxidase deficiency demonstrated consistent oxygenation during exercise indicating an under utilization of delivered oxygen. Patients with MELAS and MERRF syndrome showed similar under utilization of oxygen during exercise. Non-invasive tissue oximetry during exercise demonstrates specific abnormalities in a variety of mitochondrial disorders, indicating abnormal oxygen utilization, and will be a useful addition to the clinical investigation of such disorders.