A measles epidemic with 2255 reported cases occurred in Poland between November 1997 and July 1998, despite high vaccination coverage since the 1980s. Cases occurred at all ages less than 30 years but showed two distinct peaks: young, unvaccinated children born in 1996-1997 and once vaccinated young adults born 1976-1982. The 60% of cases were among persons aged 15 years or more. A cohort study was used to investigate measles vaccine efficacy. The efficacy of a single dose of vaccine exceeded 90% and the efficacy of two doses exceeded 99% in all age-groups. These results demonstrate both the high efficacy of the monovalent measles vaccines used in Poland and the benefit of a second dose of vaccine.