Reye's syndrome is an acute non-inflammatory encephalopathy with fatty degeneration of inner organs. The disease seems to be the metabolic response to a generalized mitochondrial insult. From 1982 to 1987, we studied 7 patients, age range being 2 months to 12 years, affected by R.S. None of them had taken salicylic acid before; three of them had been treated with acetaminophen. Two out of the seven patients examined had serious neurologic problems; other two recovered completely both under clinical and histological aspects, as it is shown by the hepatic biopsy done before and after the disease, and the remaining three died. We have tried to demonstrate the importance of hepatic biopsy in the diagnosis of R.S. and in the verification of the reversibility of the histological phenomenon. Hepatic biopsy demonstrated that the disease stopped at the initial stage, limited itself and evolved towards a complete recovery.