Objective To explore the clinical features of mitochondrial encephalomyopathy lactic acidosis and strokelike episodes (MELAS) syndrome in fratemal twins brothers.Methods The clinical data,the results of laboratory examinations,electroencephalogram (EEG),imaging,and gene detection,and the process of diagnosis and treatment were retrospectively analyzed the fraternal twin brothers with MELAS syndrome.Results The proband,a 7-year-old male,had intermittent headaches,vomit and twitching at onset.He suffered from exercise intolerance,fatigue,accompanied by short stature and hairy.The fasting blood lactic acid level was increased.Multiple video EEG showed the slowdown of background activity.Head MRI showed recurrent lesions with the characteristics of migration and variation.The point mutation rate of mtDNA A3243G was 34.7％.The diagnosis of MELAS was confirmed.At the same time,his fraternal twin brother was screened and found that his point mutation rate of A3243G was 30.0％.Although there was no clinical symptom at that time,he was onset with convulsion after 3 years.Conclusions Gene detection and family screening are helpful for the early diagnosis of MELAS.The mutation rate of A3243G is very high,which can cause an early onset and serious clinical symptoms.