Point mutation of mitochondrial DNA has been described in the blood from a MELAS patient. The 39-year-old patient developed progressive dementia, stroke-like episodes, heart conduction defect (Lown-Ganong-Levin syndrome) and cortical blindness. CT scan revealed brain atrophy and low density areas in the bilateral occipital lobes. Laboratory tests showed hyperglycemia and lactic acidosis. Muscle biopsy showed ragged red fibers on Gomori trichrome staining. He was clinically diagnosed as having MELAS and insulin-dependent diabetes mellitus. Onset of diabetes mellitus and MELAS was almost same. Family history showed his mother's brother and sisters had also insulin-dependent diabetes mellitus. We amplified the leucine (UUR) tRNA gene from the patient's blood with polymerase chain reaction (PCR) and analysed it by restriction enzyme analysis and sequencing. Genetic analysis showed A-to-G substitution at the nucleotide position 3243 in the leucine (UUR) tRNA gene. This substitution made a new restriction site Apa I. Mutant DNA coexisted with wild type DNA (heteroplasmy). It is shown that in some types of mitochondrial encephalomyopathies, especially patients of Kearns-Sayre syndrome (KSS), diabetes mellitus is often complicated. And in KSS patients insulin receptor in normal, but insulin secretion from beta cells of pancreas is decreased. In MELAS patients, however, has diabetes mellitus been reported to be rarely complicated and relationship between MELAS and diabetes mellitus is not done. As far as we know, two cases, including ours, with genetically diagnosed MELAS have been reported to have diabetes mellitus.(ABSTRACT TRUNCATED AT 250 WORDS)