Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) are common types of mitochondrial encephalomyopathy. The involved muscular pathology is characterized by typical changes of mitochondrial abnormalities. Gene screening has been the gold diagnostic standard for MELAS diagnosis. This study presents three primary MELAS patients, with an age of onset from 13 to 18 years, including one patient with seizure, and two with headache and vomiting. All patients had a family history of disease, with maternal inheritance. Cerebral magnetic resonance imaging revealed abnormally high signals in T2-weighted images: temporal lobe in three cases, occipital lobe in two cases, and parietal lobe in one case. Migrating stroke-like lesions were confirmed in one patient. Muscle biopsy revealed several strongly succinate dehydro-genase-reactive vessels scattered in muscle sections of three patients, but ragged-red fibers and cytochrome c oxidase-negative/dense (COX-/+) fibers were not observed. Mitochondrial DNA A3243G mutation was identified in all three cases. MELAS syndrome has obvious clinical heterogeneity, and muscle weakness was not prominent in some of the cases. Muscle pathological changes did not accompany ragged-red fibers or COX-/+ fibers, but succinate dehydrogenase-reactive vessels are important for MELAS diagnosis. Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) are common types of mitochondrial encephalomyopathy. The involved muscular pathology is characterized by typical changes of mitochondrial abnormalities. This screening presents have been the gold diagnostic standard for MELAS diagnosis. primary MELAS patients, with an age of onset from 13 to 18 years, including one patient with seizure, and two with headache and vomiting. All patients had a family history of disease, with maternal inheritance. Cerebral magnetic resonance imaging revealed abnormally high signals in T2-weighted images: temporal lobe in three cases, occipital lobe in two cases, and parietal lobe in one case. Migcle stroke-like lesions were confirmed in one patient. Muscle biopsy revealed several strongly succinate dehydro-genase- reactive vessels scattered in muscle sections of three patients, but ragged-red fibers and cytochrome c oxidase-negative / dense (COX - / +) fibers we Muscle pathological changes did not accompany ragged-red fibers or COX - / + fibers. mitochondrial DNA A3243G mutation was identified in all three cases. MELAS syndrome has obvious clinical heterogeneity, and muscle weakness was not prominent in some of the cases. , but succinate dehydrogenase-reactive vessels are important for MELAS diagnosis.