An unusual case of panencephalitis in a 4-year-old Japanese boy, with onset at three months after measles infection and rapid progression to a comatose state in approximately one month, is described. A rapid rise in serum measles antibody titre after the onset of the symptoms, and the appearance of various abnormal antibodies in the serum, were noted. Pathologically, the brain showed sclerosing polio- and leucoencephalitis with diffuse gliosis and sporadic intranuclear inclusions. The process is suggested to be intermediate or transitional between acute measles encephalitis and SSPE.