Four cases of mumps were reported among 180 army recruits who had received MMR vaccine 16 days earlier. Mumps serology, salivary mumps IgM and PCR tests for the SH gene were performed on the 4 cases and on 5 control recruits who remained well. PCR products were sequenced and the sequences compared to those of wild type and vaccine strains of mumps. Further salivary mumps IgM tests were performed on the remaining 171 recruits. Mumps infection was confirmed in the 4 cases but not in the 5 controls. The controls had serological evidence of prior immunity. The SH gene sequence found in the 4 cases was wild type. Saliva tests identified 2 additional recruits with mumps IgM, one of whom had presented with suspected mumps 2 days before the MMR vaccine was given. Thus 6 (5 symptomatic and 1 asymptomatic) cases of mumps in army recruits recently receiving MMR vaccine were not due to the vaccine but to coincidental infection with wild-type mumps virus. The probable index case was revealed by salivary mumps IgM tests. This study highlights the importance of appropriate investigation of illness associated with MMR vaccination.