BACKGROUND AND OBJECTIVE: Adenovirus is a common pathogen in the pediatric population. Respiratory, gastrointestinal, or renal systems are often involved in adenovirus infections. Several neurologic syndromes have been attributed to adenovirus, such as adenovirus aseptic meningitis, myelitis, subacute focal encephalitis, and Reye-like syndrome. The purpose of this study was to describe the clinical features and encephalography findings in 7 infants treated in our center for a syndrome of transient encephalopathy associated with adenovirus infection. STUDY PARTICIPANTS: Three females and 4 males ages 7 to 34 months seen in our department between July 1983 and February 1984 and September 1998 and May 1999 presented with fever of at least 7 days' duration and a gradual decline in the state of alertness. Score on the Glasgow Coma Scale ranged from 9 to 12. Findings on lumbar puncture were normal. In all 7 patients, the encephalogram showed moderate to severe background slowing compatible with encephalopathy. All patients were catarrheal and had mild hepatomegaly with slight elevation of liver enzymes. Some had bronchopneumonia, diarrhea, and conjunctivitis either isolated or in combination. METHODS AND RESULTS: Adenovirus was isolated by immunfluorescence technique in all patients-from the sputum in 3 patients, nasopharynx in 5, conjunctiva in 4, and rectal swab in 5. In 5 patients, serotyping was performed by an antibody neutralization method. Adenovirus type 3 was ascertained from a nasal swab in 1 patient, sputum specimens in 3, throat swab in 3, and rectal cultures in 5. The clinical course was characterized by a progressive recovery of alertness. After several days, there was a complete reversal of neurologic findings. CONCLUSION: We suggest that this syndrome of transient encephalopathy is a distinct entity and should be considered as another of the several neurologic syndromes known to be associated with adenovirus infection.