ObjectiveThe Canadian Acute Respiratory Illness and Flu Scale (CARIFS) was developed to measure illness severity in children with acute respiratory infection. The objective of this study was to evaluate its performance in a European primary care setting. Study design and setting178 children (median age 3 years) with cough and fever were recruited in UK general practice. Perceived severity of illness at recruitment was recorded by parents, doctors, and nurses. Parents also completed an illness diary, including the CARIF scale, until their child had recovered. In-depth interviews were conducted with 24 parents. ResultsParents found CARIFS relatively easy and quick to complete (78% of parents returned a fully completed diary covering the duration of the illness), internal consistency was high (minimum item correlation with total score 0.22; overall Cronbach's alpha statistic 0.85), and responsiveness to improvement in health was good (observed effect size of 0.45 at 8 h). At presentation, however, neither the overall CARIFS score nor the clinical element of the score correlated with physician assessment of clinical severity. ConclusionOf the three recognized domains of illness severity, CARIFS appears to be a good and valid measure of functional severity and burden of illness to the parent but it may not be a good measure of physiological severity.