Members of the peroxisome proliferator-activated receptor gamma coactivator (PGC) family are potent inducers of mitochondrial biogenesis. We have tested the potential effect of increased mitochondrial biogenesis in cells derived from patients harboring oxidative phosphorylation defects due to either nuclear or mitochondrial DNA mutations. We found that the PGC-1alpha and/or PGC-1beta expression improved mitochondrial respiration in cells harboring a complex III or IV deficiency as well as in transmitochondrial cybrids harboring mitochondrial encephalomyopathy lactic acidosis and stroke A3243G tRNA((Leu)UUR) gene mutation. The respiratory function improvement was found to be associated with increased levels of mitochondrial components per cell, although this increase was not homogeneous. These results reinforce the concept that increased mitochondrial biogenesis is a promising venue for the treatment of mitochondrial diseases.