The medium-chain acylcoenzyme-A dehydrogenase enzyme (MCAD) is involved in the initial stages of breakdown of medium-chain-length fatty acids in mitochondria. A deficiency of this enzyme can become symptomatic during a catabolic state and may resemble Reye syndrome. We report the case of a 17-month-old white girl whose 1-day history of vomiting and agitation followed by generalized unresponsiveness caused her to be brought to a local emergency department. She died after resuscitative efforts failed, and an autopsy revealed severe cerebral edema, marked hepatic steatosis, and steatosis of the renal tubule epithelia. Electron microscopy of the liver showed cytoplasmic lipid spherules and mitochondria with dense matrices, changes similar to Reye syndrome; however, postmortem examination of the decedent's blood revealed elevated medium-chain-length acylcarnitines diagnostic of an MCAD deficiency. Although uncommon, a deficiency of this enzyme should be considered in apparent Reye syndrome victims.