Here we report 2 cases of opsoclonus-polymyoclonus syndrome (OPS) associated with viral encephalitis. They had sleep disturbance, and visual hallucination. Case 1 had auditory hallucination in addition, and case 2 had dreamlike behavior. Those hallucination, which were colorful and vivid, usually appeared at the bed time. Their hallucinations were similar to peduncular hallucinations and you may also call hypnagogic hallucinations, which are often seen in patients with narcolepsy. Dreamlike behavior is observed during REM sleep in patients with brainstem damage or sometimes in the healthy elderly people. The presence of sleep disturbance, hypnagogic hallucination, and dreamlike behavior suggests that there may be some relationship between OPS and REM sleep. Considering that REM sleep is suppressed by serotonergic projection of the dorsal raphe nucleus in addition to several reports about brainstem lesion with serotonergic abnormalities in this disorder, we considered that dysfunction of serotonergic neurons of the dorsal raphe nucleus might be related in the development of OPS.