The aim of this study was to measure the seroprevalence to mumps in Norwegian conscripts belonging to the first children vaccination cohorts that had been offered two doses of MMR vaccine. The seroprevalence to mumps was 76% with the Microimmune assay and 85% with the Enzygnost assay. We also compared the performance of the Microimmune assay for detection of mumps- and measles-specific IgG antibodies in 340 paired serum and oral fluid samples from the conscripts and evaluated the effect of revaccination. Mumps-specific IgG antibodies were detected in only 61% of the oral fluids. In contrast, high levels of measles-specific IgG antibodies were detected in both the serum and oral fluid samples. Based on these results, we are only able to recommend the use of oral fluid for surveillance of measles in Norway. Our results may also indicate that the seroprevalence necessary to interrupt transmission of mumps has not been reached in vaccinated young adult Norwegians. Seroconversion was observed in all initially measles seronegative conscripts after revaccination, whereas 23 of 27 initially mumps seronegative conscripts failed to seroconvert.