A child with intractable seizures from the age of 10 months and developmental retardation developed jaundice and hepatosplenomegaly at 23 months. She died at the age of 25 months. Methionine and tyrosine were elevated in urine, plasma, CSF, and brain. These elevations were more marked in the CNS than in the blood. 4-Hydroxyphenylpyruvate dioxygenase, an enzyme involved in the metabolism of tyrosine, was undetectable in skin fibroblasts and liver. This finding together with other biochemical data suggest that our case had an inherited disorder of tyrosine metabolism, in the category of tyrosinemia I. Disturbances of tyrosine and methionine metabolism in the CNS in tyrosinemia I may be more important than has been realized. The disorder should be considered in children with unexplained epilepsy and in those who develop hepatic dysfunction while on anticonvulsants.