In early 1996, routine surveillance data raised concerns that an epidemic of mumps might be spreading to Porto from neighboring regions. A case-control study was then conducted. The objective was to test the hypothesis that the two types of vaccines—containing the Urabe or Rubini strain—provided different levels of protection during this anticipated outbreak. The mumps outbreak in Porto was not linked to insufficient vaccination coverage but to poor, or even non-existent, protection provided by the vaccine containing the Rubini strain. This study, along with others conducted in Portugal and elsewhere, supports the decision made in June 1997 by the Portuguese Ministry of Health to replace the MMR vaccine then in use (containing the Rubini strain) with a vaccine using the Jeryl Lynn viral strain, which had been proven to be more effective. By January 1998, all Portuguese health centers were using this new MMR vaccine. (R.A.)