OBJECTIVE: To assess the performance, in Brazil, of the definition of a suspected measles case among patients with rash diseases that has been adopted in Brazil and many other countries. METHODS: From January 1994 to December 2003, patients with acute rash were seen at two large primary health care units and a public general hospital in Niterói, a city in the metropolitan area of the city of Rio de Janeiro, Brazil. Data from clinical and serologic assessment were used to estimate sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of the definition of a suspected measles case that has been adopted in Brazil, as well as other combinations of signs and symptoms; serologic status was taken as the reference. Using enzyme immunoassay, serum samples were tested for immunoglobulin M (IgM) antibodies against measles virus. RESULTS: A total of 1 221 patients with an illness characterized by different combinations of rash with other signs and symptoms were studied. The suspected case definition that has been adopted in Brazil (rash, fever, and at least one of the following: cough, coryza, or conjunctivitis) had an overall sensitivity of 100%, and a specificity of 58.7%. Confirmed measles cases were 2.4 times as likely as were other rash diseases to have that combination of signs/symptoms. The suspected case definition adopted in Brazil had a 6% PPV and 100% NPV. The combination of all five signs and symptoms had the highest specificity, PPV, and likelihood ratio, for both children (< 15 years old) and adults (> 15 years). That was achieved at the expense of sensitivity, which dropped to 89%, but the NPV was still very high. CONCLUSIONS: Our results show that the suspected measles case definition adopted in Brazil is extremely sensitive for measles surveillance among patients with rash diseases. However, the high false-positive rates that were found may result in a substantial number of other rash diseases being misclassified as measles, leading to the misdirection of control measures and increases in their cost.