- Population-based seroepidemologic data on mumps have not been available in Israel since 1987, and the effects of mass immigration from the Commonwealth of Independent States during the 1990s have not been investigated. We conducted a seroprevalence study of mumps antibodies among 353 Israeli military recruits aged 18–19, based on a representative sample of sera collected in 1999. The overall seroprevalence rate was 83.3%, which was significantly lower than that measured in 1987 (94.1%, P < 0.001). Foreign-born subjects had substantially lower seroporevalence rates than their native Israeli counterparts (68.5% versus 86.1%, P < 0.001). Recent seroprevalence levels are below those required for herd immunity, and most likely contributed to an outbreak of mumps observed among young adults in Israel in 2005. Immigration appears to be a contributing factor to the decrease in population seroprevalence over time.