In Reye encephalopathy, diffuse brain swelling is a well known CT finding, but focal CT lesions have not been documented. We report 2 children with Reye encephalopathy and 2 children with non-Reye encephalopathy both associated with influenza A virus infection in whom symmetrical low density lesions of the thalamus and brainstem were detected on CT. These symmetrical low density lesions were present in the acute phase and decreased in size within a few weeks in all, and are still seen 2 years later as clearly defined small round areas in 3 surviving patients. Paired influenza A virus titers in blood showed 4-fold or more increase in all. Myelin basic protein (MBP) in CSF was increased in 2 of the 3 subjects studied. Liver biopsy showed diffuse lipid droplet infiltration in 2, focal infiltration in 1, and normal morphology in 1. The above data suggest that the symmetrical low density lesions were associated with influenza A virus infection, most likely consisting of edema, demyelination, and necrosis. We suspect that there is a continuum of Reye syndrome and virus-associated encephalopathy with significant overlap.