Disorders of mitochondrial DNA (mtDNA) may commonly present to primary care physicians but go undiagnosed. A 36-year-old man with a 15-year history of psychosis, seizures, and sensorineural hearing loss and a family history of diabetes mellitus and heart disease presented to our hospital without a unifying diagnosis. Physiologic, biochemical, and genetic testing revealed deficient aerobic metabolism, a defect in mitochondrial electron transport, and the presence of an A-to-G point mutation at position 3243 of the mitochondrial leucine-transfer RNA gene, establishing the diagnosis of mitochondrial encephalopathy, lactic acidosis, and strokelike syndrome (MELAS). Diagnosing mtDNA disorders requires a careful integration of clinical signs and symptoms with pedigree analysis and multidisciplinary testing. Diagnosis is important to provide genetic counseling, avoid unnecessary evaluation, and facilitate therapy for symptomatic relief.