In our country, there are two types of infectious mumps vaccines available. In recent times, doubts have been raised regarding the overall effectiveness of these vaccines and the comparative effectiveness of the two strains (Rubini strain and Jeryl Lynn strain). In the "East Seville" Primary Care district, 245 cases were reported in 1997 (90.1 cases per 100,000 inhabitants). This study is aimed at taking advantage of the outbreak of cases of mumps to evaluate affected populations and comparative incidence according to type of vaccines given during childhood. Descriptive analysis of the cases (age, territorial spread, inoculation history') and trend analysis (annual incident rates) within this health care district and the surrounding area. The overall effectiveness of the mumps vaccines. The case incidence rates among those inoculated with Rubini strain and those inoculated with Jeryl Lynn strain are also estimated. The highest rates of incidence are found among children in the 1-4 age range. Overall effectiveness rates for these vaccines have been estimated. A significantly higher rate of infection has been found among the children inoculated with Rubini strain than those inoculated with the Jeryl Lynn strain (relative risk of 6.5 with a Confidence Interval of 95% 3.6-11.8). The effectiveness which follows from this study does not seem as good as the theoretical effectiveness anticipated for the mumps vaccines. It thus seems advisable for other case studies to be conducted by types of vaccines used. The data to be furnished by means of sero-epidemiological studies are also of major interest.