Measles incidence has declined significantly in the United States since the 1989-1991 resurgence. Several conditions, including pockets of underimmunization, international importation, and the inability to rapidly detect and contain cases, represent potential threats to this success. During the 1995-1996 winter holiday season, the Minnesota Department of Health investigated an outbreak of measles among unvaccinated young adults affiliated with a religious community. A total of 26 outbreak-associated cases of measles were identified; most case patients (65%) were 20-29 years of age (range, 18 months to 35 years). Although case patients had multiple opportunities to expose the general public, no subsequent transmission was identified despite extensive surveillance efforts. A measles seroprevalence survey of 508 Minnesota blood donors aged 20-39 years was conducted; 91% had serologjcal evidence of immunity to measles. Our findings illustrate that high levels of population immunity can prevent transmission of measles, despite multiple opportunities for exposure.