Autoimmunity is thought to lead to islet-cell-antibody (ICA) formation in diabetes mellitus. However, we found a mitochondrial gene mutation at nucleotide pair 3243 in 3 of 27 Japanese ICA-positive, initially non-insulin-dependent diabetic patients. All 3 progressed to insulin-dependency within 13-31 months, whereas 5 of the other 24 are non-insulin-dependent after 54-90 months. ICA, at least in these 3 patients, may follow gradual beta-cell destruction due to mitochondrial gene mutation, although the possibility of beta-cells with the mutation being susceptible to autoimmune destruction cannot be excluded.