BACKGROUND In March 2000, the Israel Defense Force (IDF) experienced an outbreak of rubella. No population-based rubella seroepidemiologic data were available for the 13-year period preceding the epidemic. METHODS We conducted a population-based seroprevalence study of rubella antibodies among 289 IDF recruits drafted in 1999. RESULTS We found that 69.2% of males and 90.8% of females in service were seropositive at recruitment in 1999, immediately before the outbreak. These rates were significantly lower than those last measured in 1987 (88% among males and 98.1% among females). CONCLUSIONS Our findings indicate that this rubella outbreak among young adults was facilitated by a decrease in immunity to levels below those required for herd immunity, at which epidemic virus transmission was no longer blocked. This is most likely due to widespread pediatric vaccination coverage with incomplete catch-up immunization among adolescents and young adults. These findings serve as a reminder that changes to childhood vaccination programs may affect the epidemiology of disease among older segments of the population several years after the change is implemented.