Surveillance for measles must be enhanced to support the objective of measles elimination in the United States. Many conditions produce febrile rash illnesses that are clinically similar to measles; investigations of measleslike illnesses (MLIs) should occur regardless of the incidence of measles. Few data exist regarding the incidence of MLI in the United States, and it is unknown how providers evaluate patients with such conditions. We searched databases at a large managed care organization to obtain complete ascertainment of MLI during 1994-1998. Among 6,000,000 patient encounters, 68 records were identified that met the study case definition. The incidence of MLI was 4.5 cases/100,000 persons/year. Measles diagnoses were considered by physicians for 9 patients (13.2%); 2 were laboratory-tested and reported to the state health department and the other 7 were given alternative diagnoses. It was not possible to determine for the remaining MLI patients whether measles was ruled out on clinical grounds or whether the possibility was not considered. Provider education regarding evaluation and reporting of measles is warranted. Databases at health care plans can be used to address public health issues and to establish syndromic surveillance for communicable diseases.