Peroxisome proliferator-activated receptor-γ co-activator-1α (PGC-1α) is a key nuclear receptor co-activator for mitochondrial biogenesis. Here we report that overexpression of PGC-1α in skeletal muscles increased mitochondrial number and caused atrophy of skeletal muscle, especially type 2B fiber-rich muscles (gastrocnemius, quadriceps, and plantaris). Muscle atrophy became evident at 25 weeks of age, and a portion of the muscle was replaced by adipocytes. Mice showed increased energy expenditure and reduced body weight; thyroid hormone levels were normal. Mitochondria exhibited normal respiratory chain activity per mitochondrion; however, mitochondrial respiration was not inhibited by an ATP synthase inhibitor, oligomycin, clearly indicating that oxidative phosphorylation was uncoupled. Accordingly, ATP content in gastrocnemius was markedly reduced. A similar phenotype is observed in Luft's disease, a mitochondrial disorder that involves increased uncoupling of respiration and muscle atrophy. Our results indicate that overexpression of PGC-1α in skeletal muscle increases not only mitochondrial biogenesis but also uncoupling of respiration, resulting in muscle atrophy.