A child with acute infantile lymphoblastic leukaemia in remission after cytostatic therapy contracted apparently uncomplicated measles. 8 weeks later, an acute neurological syndrome developed, which led to death within 2 weeks. At autopsy, a peculiar type of inclusion body "encephalitis" almost without signs of inflammatory infiltration was found. Electron microscopic examination showed changes of paramyxovirus infection, which were attributable to slow measles virus disease of the CNS. There was evidence of disturbed nucleocapside synthesis, which is considered to be one of the basic defects in the development of slow-virus processes and their altered viral maturation. Possible pathogenetic factors are discussed in the light of our case.