Clinical, electroencephalographic, and other laboratory features and subacute sclerosing panencephalitis (SSPE) were studied in eight patients by the same group of investigators. The relatively high frequency of the disease within a small geographical area in a short period of time suggests that SSPE may be unusually common in the vicinity of Memphis. The delineation of four clinical stages of the disease, markedly elevated measles antibody titers in serum and cerebrospinal fluid, ultrastructural observation of intranuclear and intracytoplasmic inclusions in neurons and neuroglia in three patients, and alteration of serum and CSF immunoglobulins indicate that the measles myxovirus may be the etiological agent in these patients.