We have in a longitudinal study determined the proportion of the mitochondrial A3243G mutation in DNA obtained from cervical cell samples collected from three individuals affected with mitochondrial diabetes and hearing loss during a period of up to 18 years. Using the minisequencing method we were able to sensitively determine the proportion between mutant and normal mitochondrial DNA. Our results demonstrate a constant decrease in the levels of the pathogenic mutation in mitotic tissues of affected individuals with time.