Sir.—I would like to comment on the article by Pichichero and McCabe as found in the Journal (132:1097-1099, 1978). Centers concerned with the treatment of Reye's syndrome have all seen one or two cases of "recurrent Reye's Syndrome"1 since the disease has been described in 1963. The concept is certainly important since it implies either an inherent susceptibility to the disease of a recurrent exposure to an as yet undefined toxin. The importance of this concept cannot be overstated and the authors emphasize this in their last paragraph: "Other diagnoses must be excluded including foreign travel, toxic ingestions, certain viral encephalitides, hypovolemic shock, the female carrier state for ornithine transcarbamylase deficiency and systemic carnitine deficiency." At the present time, the only clear-cut way to distinguish Reye's syndrome from other encephalopathies with associated liver dysfunction is to establish abnormal mitochondrial morphology by electron microscopy and mitochondrial dysfunction by light histochemistry.