MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis and strokelike episodes) is a distinct disorder characterized clinically by repeated strokelike attacks mostly beginning in childhood. We have paid special attention to the blood vessel abnormality seen in most biopsied muscle, in terms of the strokelike episodes in MELAS. The 3243 mutation in 80% of the typical MELAS patients has also been found in patients differing from the MELAS phenotype. Because we have examined muscle biopsies in 94 MELAS or 3243-positive patients, it is worthwhile to summarize the clinical and pathological findings and to prove the discrepancy between phenotype and genotype. This may be a starting point for further discussion of the pathomechanism and so toward further understanding of the disease itself.