A cluster of nine severe local reactions occurred in a Pennsylvania school district following a mass measles immunization program in February 1981. A case-control study showed that the nine cases had a statistically significant increased history of having received killed measles vaccine previously (p less than .001, Fisher's exact test) and a statistically significant higher geometric mean measles antibody titer (p less than .01, Student's t test) than controls, suggesting that previous receipt of killed measles vaccine was the underlying cause of these reactions.