A 5-year-old white boy with X-linked hypogammaglobulinemia and subacute encephalitis developed paresis of the right upper and lower extremities, symmetrical polyarthritis, and bilateral basal bronchiectasis at 1 year of age. At 5 years, he was exposed to rubeola and received a preventive dose of γ-globulin. During the next six months he developed progressive mental and motor deterioration. Electroencephalographic pattern was consistent with subacute sclerosing panencephalitis. Brain biopsy revealed extensive inflammatory infiltrates but neither inclusion bodies nor viral particles could be demonstrated by electron microscopy. Immunofluorescence studies of the biopsy specimen and the ethanol fixed monolayer cell cultures revealed intracellular fluorescence specific for measles antigen. As expected, serologic studies for measles antibodies were negative. On the basis of clinical, EEG, and histopathologic studies, in the absence of measles antibodies in serum and cerebrospinal fluid, we chose the term subacute encephalitis rather than subacute sclerosing panencephalitis.