Urinary excretion of the organic acids in patients with type I and III glycogenosis was investigated. In all patients with type I glycogenosis, urinary α-ketoglutarate concentration was about 10 times the normal value. α-Ketoglutaric aciduria was not improved by the acute or prolonged administration of a large dose of cofactors for pyruvate and α-ketoglutarate dehydrogenase complex. On the other hand, the level of α-ketoglutarate in the urine from type I patients decreased in conjunction with the decrease of plasma lactate and pyruvate concentration after repeated oral glucose loading. Oral citrate loading brought an increased excretion of α-ketoglutarate in type I glycogenosis. It is possible that α-ketoglutarate dehydrogenase is the rate-limiting step in tricarboxylic acid cycle and in patients with glycogenosis type I, the excessive excretion of α-ketoglutarate may be caused by the limited activity of α-ketoglutarate dehydrogenase with excessive substrate.