Background & Aim: Mumps infection is endemic in Iran. Due to high percentage of unvaccinated young people(10y(2.1%). Acute mumps infection(positive IgM) was seen in 8.3% and previous immunity to mumps(IgG) in 24.1% of all. The highest incidence of acute infection(IgM) was in 3-5-year old group, but it was not different in other age groups(P=0.11). Previous infection(IgG was also higher in 3...