During 2004-2005, the United Kingdom (UK) experienced a nationwide epidemic of mumps, which peaked during 2005 when 56,390 notified cases were reported in England and Wales. The majority of confirmed cases during 2004-2005 were in persons aged 15-24 years, most of whom had not been eligible for routine mumps vaccination. Mumps usually is a self-limited viral disease that appears as parotitis. However, mumps also can lead to serious complications such as encephalitis or pancreatitis. This report summarizes the epidemiology of the 2004-2005 mumps epidemic in England and Wales.