Cell and tissue damage in respiratory chain disorders have been related to increased production of reactive oxygen species (ROS). We measured telomere lengths in such disorders since ROS have also been implicated with telomere shortening. We investigated whole blood cell DNA of 14 patients with MELAS-related mitochondriopathy and two patients with the LHON-associated G11778A mutation of the mitochondrial genome. The phenotypes were variable and included an unusual case of schizophrenia-like psychosis associated with the A3243G mutation. As compared to healthy controls telomere shortening in the patient group was advanced (P < or = 0.006). We compare this finding with the accelerated telomere shortening in Down's syndrome and in chromosomal breakage syndromes. We discuss possible relations between advanced telomere shortening and selective constraints that act on proliferating cells with respiratory chain dysfunction.