Four patients with clinically and genetically defined MELAS were examined using quantitative localized proton magnetic resonance spectroscopy of the brain. Acute and chronic lesions were located in the occipital lobe and mostly characterized by strongly elevated concentrations of lactate (Lac) and glucose (Glc) as well as severely reduced concentrations of total N-acetylaspartyl compounds (tNAA, neuroaxonal markers), glutamate (Glu), and total creatine. These findings indicate a high degree of nonoxidative glycolysis reflecting either impaired oxidative energy metabolism or the use of anerobic metabolism by infiltrating macrophages as well as damage or loss of viable neuroaxonal tissue. In contrast, glial cell populations, in particular astrocytes, seem to remain unaffected as evidenced by unchanged concentrations of myo-inositol (glial marker). In addition, all patients including one who never experienced a stroke-like episode showed elevated Lac and Glc as well as reduced tNAA and Glu in tissues appearing normal on MRI. These disturbances were stronger in cortical gray matter and cerebellum than in white matter and indicate that neuroaxonal damage is not restricted to structural lesions. The steady presence of Lac is consistent with a reduced capacity of the mitochondrial oxidative energy metabolism resulting from impaired respiratory chain function.