The objective of this study was to investigate the possibility of dengue virus infection causing an abnormal neurologic presentation. Between 1996 and 1998, all pediatric patients with clinical manifestations of encephalitis-like illness who were admitted to the Department of Pediatrics, Siriraj Hospital were prospectively studied for any evidence of dengue virus infection. The diagnosis of dengue virus infection was based on mosquito viral isolation and serologic and polymerase chain reaction (PCR) evidence. Of 44 patients with the preliminary diagnosis of acute viral encephalitis, 8 were diagnosed with dengue infection. All of these 8 patients were diagnosed by serology. In addition to the serologic diagnosis, four also had positive PCR, one had positive viral isolation, and one had both positive PCR and viral isolation. Only two patients were diagnosed by serologic evidence alone. All except one had clinical courses and laboratory findings compatible with typical dengue infection. All had obvious encephalitic clinical manifestations with normal cerebrospinal fluid findings except one patient, who had mildly increased cerebrospinal fluid protein. All of these patients recovered completely and had benign clinical courses except one patient, who developed leakage symptoms. None had liver failure. Dengue virus can cause acute encephalopathy with fever. It can masquerade as other types of acute viral encephalitis. However, its clinical course and prognosis are usually favorable.