• We performed biochemical studies on isolated mitochondria from a muscle biopsy specimen in a patient with Lowe's syndrome. Respiratory controls of mitochondrial preparations with substrates reducing nicotinamide adenine dinucleotide and with a flavoprotein-linked substrate were markedly diminished, but the oxygen consumption was normal with ascorbate and tetramethylphenylenediamine as substrates, which suggested a defect in electron transport prior to the cytochromes. The organelles also showed decreased adenosine diphosphate phosphorylate-oxygen ratio, indicating a partial uncoupling. These findings suggest that Lowe's syndrome could be considered a mitochondrial disease.