Measles has had a severe impact on children in the United States since colonial times. In the early decades of the 20th century, thousands of fatal measles infections were reported each year. During the 1950s an annual average of greater than 500,000 cases of measles and nearly 500 deaths due to measles were reported in the United States. Surveys indicated that 95% of the population had been infected with measles by the age of 15 years. The introduction of measles vaccine and its widespread use, which began in 1963, has had a major impact on the occurrence of measles in the United States. Reported numbers of cases, deaths due to measles, and complications of measles (e.g., encephalitis) have declined dramatically. Accompanying the decline in reported incidence of measles and following it by approximately seven years, has been a decline in the reported incidence of subacute sclerosing panencephalitis (SSPE). In recent years, the incidence of measles has dropped to levels that are less than 1% of those seen in the prevaccine era. In 1981, provisional figures indicated that only 10% of counties in the United States reported any cases of measles. The reported incidence in 1981 was 1.3 cases per 100,000 population, compared with an average incidence of 336.3 cases per 100,000 population in the decade 1950-1959. Thus, the impact of measles in the United States has been markedly reduced, and it is anticipated that indigenous transmission will be eliminated entirely from the country within the year.