We describe the full history and postmortem findings in one of the first identified cases of mitochondrial encephalomyopathy with stroke-like episodes (MELAS). To clarify diagnostic criteria, we analyzed 69 reported cases. The syndrome should be suspected by the following three invariant criteria: (1) stroke-like episode before age 40 yr; (2) encephalopathy characterized by seizures, dementia, or both; and (3) lactic acidosis, ragged-red fibers (RRF), or both. The diagnosis may be considered secure if there are also at least two of the following: normal early development, recurrent headache, or recurrent vomiting. There are incomplete syndromes in relatives of patients with the full syndrome and incomplete syndromes might also be encountered in sporadic cases. Some MELAS patients have features of the Kearns-Sayre syndrome (KSS) or myoclonic epilepsy with ragged-red fibers (MERRF), but none had the full KSS syndrome. In partial or confusing cases, analysis of mitochondrial DNA (mtDNA) may point to the correct diagnosis; however, not all patients with clinical MELAS have had the typical mtDNA point mutation and some patients with the mutation have clinical syndromes other than MELAS.