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BACKGROUND: Mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS) is a common syndrome of mitochondrial diseases caused primarily by a mutation from adenine to guanine at mitochondrial DNA 3243. However, the correlation between heteroplasmic mutations and clinical characteristics of hereditary MELAS syndrome is unclear. OBJECTIVE: To survey the clinical behaviors, biochemical outcomes, and imaging data in a patient with suspected MELAS syndrome by maternal inheritance, and to investigate the correlation with heteroplasmic mutations of hemocyte mitochondrial DNA. DESIGN, TIME AND SETTING: A case analysis based on hereditary family surgery was performed in the Enliang Hospital of Anshan, Taian County, and biochemical tests and gene diagnosis were erformed at the Department of Laboratory and Institute of Neurology, the First Affiliated Hospital of China Medical University, between March and September 2009. ARTICIPANTS: A 22-year-old female patient with MELAS syndrome was diagnosed in the First Affiliated Hospital of China Medical University in January, 2009. She had five males and seven females in her maternal family. METHODS: We obtained stroke and convulsion history in the patient and her family, as well as erforming routine blood tests, plasma lactic acid levels before and after movement, and magnetic resonance of the head. A mutation at m.3243A > G was verified using polymerase chain reaction-restriction fragment length polymorphism and DNA sequencing, and quantitated using real-time polymerase chain reaction. MAIN OUTCOME MEASURES: Correlation of clinical behaviors and biochemical outcomes, as well as imaging data with heteroplasmic mutations in family members with typical and atypical MELAS syndrome. RESULTS: Some family members had typical symptoms of convulsion, stroke, and MELAS syndrome, as well as atypical symptoms of microsomia, movement intolerance, febrile, and migraine. Magnetic resonance of the head was consistent with typical imaging data of MELAS syndrome during attacks, and family members showed cerebellar atrophy. A heteroplasmic mutation of mitochondrial DNA 3243 occurred in all family members, although higher levels caused severe typical symptoms. The age of first-onset convulsion was negatively correlated with level of heteroplasmic mutation (r= -0.852, P< 0.05), but lactic acid was positively correlated with mutation levels (before movement, r= 0.945, P< 0.001; after movement, r= 0.945, P< 0.001). CONCLUSION: MELAS syndrome was diagnosed in this family by maternal inheritance, and the etiological factor was a mutation of mitochondrial A3243G. The level of heteroplasmic mutation correlated with anticipated convulsion and lactic acid levels.