Oxygen extraction fraction (OEF) is defined as the ratio of blood oxygen that a tissue takes from the blood flow to maintain function and morphological integrity. OEF reflects the efficiency of oxygen utilization by the tissue and, therefore, is a hemodynamic measure in brain ischemia. Mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) is a common mitochondrial disorder. It is characterized by neurological remissions and relapses and associated with progressive neurocognitive deficits. Because of abnormalities of mitochondrial function in MELAS, defects in the oxidative metabolic pathways of energy production decrease the cerebral oxygen utilization and lead to the reduction of OEF. Quantification of OEF can reflect the functional status of cerebral mitochondria and provide insight into the pathophysiological changes in the brain in MELAS. In light of recent advances in MRI, the discovery of the blood-oxygen level-dependent signal has allowed development of MRI methods targeted toward quantitative OEF imaging. A new MR sequence, termed the gradient-echo sampling of spin echo, was successfully developed to enable quantitative assessment of the OEF in the brain tissue. MR OEF imaging in patients with MELAS detects extensive OEF reduction in the stroke-like lesions, as well as in the normal-appearing brain regions. More severe dysfunction of the mitochondria in the stroke-like lesions was implied at the onset of the stroke-like episode. Determination of OEF throughout the episode demonstrated a chronological change in mitochondrial function in individual cases. Such neuroimaging findings might provide some clues in the investigation of the underlying mechanisms of stroke-like episodes.