Increased demand for medical services and school absenteeism related to vaccine reactions following a oneday mass measles immunization program in Rhode Island were studied. Measles virus vaccine, live, attenuated (Schwarz) was used. Nearly 60% of susceptible children (30,647) were immunized. Increased demand for medical services averaged less than one telephone call per day per physician during the week when most reactions occurred. For two weeks preceding immunization, however, many physicians experienced a sharp increase in calls from parents requesting immunization histories of their children. School absenteeism among those vaccinated was higher by 3.5% throughout the state, except in certain communities where epidemic influenza B morbidity was reaching peak levels. Here, absenteeism patterns were reversed. Interferon induced in those vaccinated by the measles virus vaccine is suggested as an explanation for the apparent protective effect against influenza B.