BackgroundSeverity assessments of respiratory tract infection (RTI) in children are known to differ between parents and clinicians, but determinants of perceived severity are unknown.AimTo investigate the (dis)agreement between, and compare the determinants of, parent and clinician severity scores.Design and settingSecondary analysis of data from a prospective cohort study of 8394 children presenting to primary care with acute (≤28 days) cough and RTI.MethodData on sociodemographic factors, parent-reported symptoms, clinician-reported findings, and severity assessments were used. Kappa (κ)-statistics were used to investigate (dis) agreement, whereas multivariable logistic regression was used to identify the factors associated with illness severity.ResultsParents reported higher illness severity (mean 5.2 [standard deviation (SD) 1.8], median 5 [interquartile range (IQR) 4–7]), than clinicians (mean 3.1 [SD 1.7], median 3 [IQR 2–4], P<0.0001). There was low positive correlation between these scores (+0.43) and poor inter-rater agreement between parents and clinicians (κ 0.049). The number of clinical signs was highly correlated with clinician scores (+0.71). Parent-reported symptoms (in the previous 24 hours) that were independently associated with higher illness severity scores, in order of importance, were: severe fever, severe cough, rapid breathing, severe reduced eating, moderate-to-severe reduced fluid intake, severe disturbed sleep, and change in cry. Three of these symptoms (severe fever, rapid breathing, and change in cry) along with inter/subcostal recession, crackles/crepitations, nasal flaring, wheeze, and drowsiness/irritability were associated with higher clinician scores.ConclusionClinicians and parents use different factors and make different judgements about the severity of children’s RTI. Improved understanding of the factors that concern parents could improve parent–clinician communication and consultation outcomes.