An outbreak of mumps in a middle school (grades 6 through 8) in Ohio during 1981 was investigated to determine the effectiveness of mumps vaccine. Of the 481 middle school students on whom questionnaires were completed, 62 (12.4%) exhibited clinical mumps. The overall vaccine efficacy was 81.2% when children with a history of mumps disease are excluded from the analysis. Using a logistic regression model with the presence or absence of clinical mumps as the dependent variable, three factors were found to be significant: mumps vaccine, a history of mumps disease, and sex. Factors that did not significantly affect the rate of disease among vaccinated pupils included whether the mumps vaccine was administered singly or in combination with rubella and/or measles vaccine, age at vaccination, year of vaccination, and month of vaccination.