When an extensive measles epidemic broke out in Turku in late 1975, the availability of a previous study on measles vaccination reactions in 442 children permitted a comparative evaluation to be made on reactions to and the efficacy of measles vaccine (Schwartz strain) administered after exposure to natural measles. In a preliminary study, nine children were vaccinated one to 14 days after exposure to natural measles but before prodromal symptoms appeared. Only one of these nine children developed symptoms and signs comparable to those of the natural disease. In five day-care centers the children were vaccinated when five or more children out of 30 to 40 at each center had measles. In only five of 74 exposed children were the signs and symptoms comparable to those of natural measles. No marked differences in signs and symptoms after the vaccinations were observed between the exposed and nonexposed children. It is concluded that vaccination is safe, can usually prevent measles, even when administered after a prolonged interval following exposure to natural disease, and can usually control an epidemic in progress.