As part of most influenza surveillance systems, patients who meet specific symptom criteria will have culture samples taken for laboratory testing. (1,2) Several surveillance definitions of influenza-like illness (ILI) have been proposed. (2-6) The Centers for Disease Control and Prevention (CDC) in the United States defines ILI as the presence of fever (temperature of 38 degrees Celsius or greater) and one of either cough or sore throat or both, in the absence of a known cause other than influenza. (7) Health Canada's Flu Watch uses a variant of the CDC definition of ILI: fever and cough plus one or more of the following--sore throat, arthralgia, myalgia, or prostration (www.phac-aspc.gc.ca/fluwatch). Several studies have found that the grouping of high fever and cough is the best predictor of influenza. (8-11) What these ILI definitions have in common is the presence of fever plus one or more symptoms of respiratory illness. Data about influenza symptoms can be obtained from multiple sources. For example, symptoms can be reported by multiple informants, such as self-reports and health care providers; or by multiple methods, such as symptom checklists and medical record data. In prior work, we found that agreements between health record data and self-report varied for respiratory-related symptoms. (12) Therefore, factors that might influence the sensitivity, specificity and predictive values of ILI include the actual surveillance definition, but also the data source from which the symptom data contained in the ILI definition are taken. The impact of these factors will be relevant to both public health researchers and clinicians in determining choice of ILI definitions.