By quantitative polymerase chain reaction (PCR) of total cellular DNA, the known 4977 bp deletion in human mitochondrial DNA (mtDNAΔ4977) was not detected in rapidly dividing tissue such as placenta and lymphocytes, nor in brain tissue from fetuses and in frontal cortex from two individuals 24 and 56 years old. However, in frontal cortex from individuals 71–95 years 0.13% deleted/undeleted mtDNA was found, with no significant difference between Alzheimer patients (0.14%) and age-matched controls (0.12%). We hypothesize that the age-related accumulation of this deletion (and other expected deletions) contributes to the down-regulation of adenosine triphosphate (ATP) production in neurons and other nondividing cells, a fundamental mechanism common to aging and Alzheimer's disease.