The authors describe a newborn with postnatal myopathy who subsequently developed feeding difficulties, ophthalmoplegia, ptosis, encephalopathy, and seizures. She became ventilator dependent after sudden apnea. The myopathy was without ragged red fibers in the muscle biopsy. An electron transport chain study showed a markedly generalized low level of enzyme activity, particularly in complexes I, I + III, and IV. An initial electroencephalogram finding was normal; subsequent electroencephalograms showed suppression bursts. The mitochondrial copy number in skeletal muscle was 2% of normal.