Purpose: The introduction of vaccine has dramatically reduced the number of mumps patients, but outbreaks have recently occurred among highly vaccinated populations in developed countries. Patients with mumps admitted between 1989 and 2012 observed in a single hospital in Korea are described in the present study. Methods: We retrospectively analyzed the inpatients with mumps between 1989 and 2012 and the outpatients in 2011-2012. Results: A total of 152 mumps patients were admitted between 1989 and 2012, and 163 outpatients were recorded in 2011-2012. The highest number of admitted cases occurred in 1998 (35 cases) and 2012 (34 cases). In outpatients, the age distribution showed the highest frequency among people 15 to 19 years of age and rare frequencies in those 4 years and ＞ 20 years of age, which was compatible to the data of our city and national data. When comparing the patients in 1998 to those in 2010-2012, there were significant differences regarding the mean age and the rate of secondary measles-mumps-rubella (MMR) vaccine, but clinical features including complications were similar, except aseptic meningitis. Anti-mumps IgG was positive in 83% and 100% of patients in the two time periods, and IgM was positive in 67% and 41%, respectively. Conclusions: Recently in Korea, mumps outbreaks are occurring in mainly secondary school students who received two-dose of MMR vaccine. The vaccineses might have a modified immune reaction to viral insults as well as a modified clinical manifestation.