Adult onset Leigh syndrome with a nucleotide (nt) 8993 mutation in mitochondrial (mt) DNA is reported. A 43-year-old woman with a 6-year-history of insulin-resistant diabetes mellitus developed muscular weakness, intractable nausea and vomiting, and anemia. These were followed vertigo, blindness, and deafness with nystagmus. Magnetic resonance imaging (MRI) revealed abnormal high intensities in the bilateral medial regions of the thalamus and periaqueductal gray matters. Autopsy disclosed well-demarcated necrotizing lesions with prominent capillaries in the areas detected by MRI, which were sufficiently diagnostic for Leigh syndrome. MtDNA analysis performed on DNAs extracted from formalin-fixed tissues including liver, heart, brain, muscle, kidney and pancreas showed a T-->G mutation at nt 8993. This is the first case of adult Leigh syndrome demonstrating on mtDNA mutations.