Although adenovirus type 3 is a common pathogen of pediatric infection, there have been few reports on encephalitis and encephalopathy caused by this virus. We report two cases, one of acute encephalitis and another of transient encephalopathy, associated with adenovirus type 3 infection. Case 1, an 11-year-old boy with unconsciousness and convulsions, was diagnosed as having acute encephalitis because adenovirus type 3 was isolated from the cerebrospinal fluid. Case 2, a 3-year-old boy with intermittent excitement and hallucinations, was diagnosed as having transient encephalopathy. MRI, EEG, and cerebrospinal fluid studies were normal. Our cases and previously reported cases exhibit a wide clinico-pathological spectrum of the central nervous system involvement by adenovirus type 3.