This manuscript summarizes our studies on mitochondrial DNA and enzymatic abnormalities that accumulate, with age, in skeletal muscle. Specific quadricep muscles, rectus femoris in the rat and vastus lateralis in the rhesus monkey, were used in these studies. These muscles exhibit considerable sarcopenia, the loss of muscle mass with age. The focal accumulation of mtDNA deletion mutations and enzymatic abnormalities in aged skeletal muscle necessitates a histologic approach in which every muscle fiber is examined for electron transport system (ETS) enzyme activity along its length. These studies demonstrate that ETS abnormalities accumulate to high levels within small regions of aged muscle fibers. Concomitant with the ETS abnormalities, we observe intrafiber atrophy and, in many cases, fiber breakage. Laser capture microdissection facilitates analysis of individual fibers from histologic sections and demonstrates a tight association between mtDNA deletion mutations and the ETS abnormalities. On the basis of these results, we propose a molecular basis for skeletal muscle fiber loss with age, a process beginning with the mtDNA deletion event and culminating with muscle fiber breakage and loss.