Of the 75 patients with infection-related acute encephalopathy or encephalitis treated by us in the last 10 years, 28 had acute onset encephalitis. The results of clinical studies on these 28 patients were as follows: (1) The number of cases who exhibited CNS manifestations during (A) and after pyrexia (B) were 15 and 13, and the ages of predilection were infancy and school age, respectively. (2) MRI studies in cases of A revealed multifocal CNS lesions in 1 case, localized lesions in 7 and normal findings in 7. In B, there were 3 cases with multifocal lesions, 8 of focal lesions and 2 of normal findings. (3) The 7 cases of localized lesions in A were divided into 5 of herpes encephalitis and 2 of suspected vasculitis. Vasculitis was suspected in 3 of 8 cases of localized lesions in B. Thus, vasculitis is considered to be an important cause of encephalitis. (4) Brain lesions in the 5 cases of herpes encephalitis were occipital dominant in 4. Only one case had a temporal lesion. (5) All cases with focal MRI lesions and CSF pleocytosis, having evidence of direct viral invasion, were herpes encephalitis. Direct viral invasion was not proven in any other cases. (6) Although the term encephalitis is often used in clinical practice, the process by which the CNS lesions occur in acute viral infection is still unknown. Therefore it is not easy to establish the diagnosis. The diagnostic criteria of encephalitis should be reconsidered. New specific methods to analyze the cause of CNS lesions are necessary.