To determine the relationship between mitochondrial DNA (mtDNA) mutations and age-related impaired glucose tolerance (IGT), mtDNA from skeletal muscle of 19 volunteers, ages 55-75 years, with either IGT or diabetes and 17 age- and sex-matched control subjects was analyzed using a long-extension polymerase chain reaction (PCR) combined with a quantitative PCR. We found the common 4,977-bp deletion in 84% of the IGT/diabetes group compared with only 41% in the control group (P < 0.02). Multiple other deletions of different sizes were identified in 13 out of 19 IGT/diabetes patients (68%) compared with 2 out of 17 control subjects (12%) (P < 0.002). Because of the heterogeneity and variation in the mutations identified, we propose that these mtDNA mutations were the result rather than the cause of IGT. The increase in type and frequency of mtDNA deletions in diabetes and IGT patients may be related to oxidative damage by oxygen free radicals. These may be produced in greater amounts as a result of hyperglycemia or may be more abundant because of an abnormality in the scavenging of free radicals by antioxidants.