An outbreak of measles at the University of California at Los Angeles provided the opportunity to study clinical, epidemiologic, and serologic characteristics of the disease in young adults in the present vaccine era. Of the 34 cases studied, 18 occurred in persons who thought they were immune. Fifteen of 19 seronegative students vaccinated during the epidemic responded with a secondary (IgG) antibody response. Antibody prevalence studies indicated that 91% of the student population had measles antibody at the onset of the outbreak, and history relating to measles correlated poorly with antibody prevalence. Of 212 adults vaccinated, 58% complained of one or more symptoms. Seventeen percent were confined to bed, and in three women vaccine-associated illness was notably severe. That measles will continue to be a problem in adults with our present national approach to immunization is predicted.