The impact of a mass vaccination campaign against measles-rubella in England and Wales was assessed using the results of a saliva test for measles-specific IgM, which was offered to all notified cases of measles. By means of clinical data supplied by the reporting doctors, we estimated the sensitivity and specificity of various clinical case definitions in predicting a confirmed case. A saliva sample was obtained within the appropriate time period for 3442/7574 (45.4%) of notified cases; the proportion confirmed by saliva testing was low and fell from 67/681 (9.8%) at the start of the campaign to 1/373 (0.3%) after 35 weeks. The specificity of all clinical case definitions was low, but was highest at 41.1% (95% confidence interval, 39.2-43.0%) for the definition similar to that recommended in the USA. Extrapolating from cases observed during ongoing surveillance permits estimation of an annual notification rate for nonmeasles rash and fever illness. For countries with good measles control, a clinical case definition is too nonspecific for accurate surveillance. Laboratory confirmation of suspected measles cases is required and saliva testing is an acceptable method. The continued sensitivity of measles surveillance in England and Wales requires that the incidence of notified rash and fever illness and the proportion of cases in which measles infection was excluded by laboratory testing be monitored.