We investigated clinical features of juvenile patients presenting non-herpetic viral acute encephalitis (4 men and 7 women, aged of onset; 23.7 +/- 3.3 years) without malignancy and immunodeficiency. We divided the patients into two groups according to initial neurological symptoms: psychiatric symptoms mimicking schizophrenia (group P, n=5), seizure (group S, n=6), and compared clinical manifestations among the two groups. Symptoms frequently seen in initial phase of the illness were neck stiffness (4 cases, 36%), involuntary movement (7 cases, 64%) and convulsion (8 cases, 73%). There were no significant difference among the groups except seizure. Patients in group P had more CSF cells and CSF lymphocytes compared with other groups (p < 0.05 and p < 0.01, respectively). Abnormal intensities in T2-weighted magnetic resonance images were found in 4 cases (36%). The term from the onset to leaving hospital of group P (213 +/- 227 days) was longer than that of group S (98 +/- 85 days), although it did not reach a significant difference. These findings indicate that juvenile acute non-herpetic encephalitis initially presenting psychiatric symptoms was serious and had relatively poor prognosis.