The occurrence of cardiomyopathy in the course of several mitochondrial disorders has been previously reported (Papadimitriou et al 1984; DiMauro et al 1985). On the other hand, a high proportion of isolated cardiomyopathies remains unexplained. Indeed, while asymmetric hypertrophies have occasionally been ascribed to an autosomal dominant gene mapping to chromosome 14 (Jarcho et al 1989), the cause of concentric cardiomyopathies remains largely unknown. This prompted us to investigate the potential use of endomyocardial biopsies for the early detection of oxidative phosphorylation defects in 6 patients with concentric cardiomyopathy.