Type I hyperoxaluria results from reduced activity of α-ketoglutarate: glyoxylate carboligase, which is necessary for the synergistic decarboxylation of glyoxylate and α-ketoglutarate to α-hydroxy-β-keto-adipate. Since thiamine pyrophosphate is a cofactor in the reaction, thiamine deficiency might be expected to result in tissue oxalosis. However, there was no significant increase in the incidence of renal oxalosis in 15 patients with Wernicke's encephalopathy at necropsy compared with controls. It is possible that hyperoxaluria was present in these thiamine-deficient patients but at a urine concentration below that necessary for calcium oxalate deposition. It is also possible that the severity of the thiamine deficit required for hyperoxaluria exceeds that for the neuronal and cardiac manifestations.