The authors analyzed data from hospital admissions and enhanced mumps surveillance to assess mumps complications during the largest mumps outbreak in England and Wales, 2004–2005, and their association with mumps vaccination. When compared with non-outbreak periods, the outbreak was associated with a clear increase in hospitalized patients with orchitis, meningitis and pancreatitis. Routine mumps surveillance and hospital data showed that 6.1% of mumps patients were hospitalized, 4.4% had orchitis, 0.35% meningitis and 0.33% pancreatitis.