Although the United States has enjoyed great success in the control of mumps since the licensure of live virus mumps vaccine in 1967, a relative resurgence of mumps during the 1986-1987 period has raised concerns about the long-term effectiveness of mumps vaccine. We received mumps surveillance data, historical information on mumps vaccine distribution and recommendations for its use, survey data on levels of mumps immunization in US children during the 1973-1985 period, the effect of mumps immunization school laws on the reported incidence of mumps among states with and without such laws, and studies of mumps vaccine effectiveness. Following licensure, a decade elapsed before mumps vaccine was endorsed as a routine immunization of childhood, while immunization survey and vaccine distribution data reflected only gradual acceptance of the vaccine. However, mumps incidence declined during this period, resulting in a relatively underimmunized cohort of children born between 1967 and 1977 who grew up during a period when the risk of exposure to mumps was rapidly declining. The resurgence of mumps since 1986 has been characterized by a selective increase in incidence and a shift in the age group at highest risk to middle and high school students in those states lacking comprehensive mumps immunization school laws. Postlicensure field evaluations of mumps vaccine effectiveness have not demonstrated waning vaccine-induced immunity. The data indicate that the relative resurgence of mumps in the United States is chiefly due to a failure to vaccinate all susceptible persons, especially those who are now between 10 and 19 years old. Mumps immunization school laws offer an approach to deal with the problem of continuing susceptibility in school-age populations.