Highly immunogenic, safe, and effective attenuated measles vaccines, stabilized to lengthen duration of potency under field conditions, are available. Measles can be controlled while studies continue to define the molecular change in the viral genome responsible for attenuation and to clone the genes that specify for the two major surface glycoprotein antigens, the hemagglutinin (HA) and fusion (F) proteins. Purification of HA and F antigens will facilitate studies of cell-mediated immunity. Elucidation of host cell-dependent lack of synthesis of the internal membrane protein in the brains of patients with subacute sclerosing panencephalitis (SSPE) is needed. Control of measles will clarify the relationship between acute measles and those chronic neurologic diseases known (e.g., SSPE) or thought (e.g., multiple sclerosis) to be caused by persistent infection with measles virus. Epidemiologic studies should determine the optimal age for immunization as age-specific morbidity and mortality are reduced and should record the subsequent course of apparent vaccine failures in children immunized from seven to 15 months of age. Behavioral and operational research must examine how best to extend measles immunization throughout the world.