National guidelines recommend the use of clinical history and spirometry to determine asthma severity. We examined the usefulness of the six guideline-recommended clinical questions in determining asthma severity and then compared guideline-determined severity to clinician-reported and spirometry-determined severity in a cross-sectional study of 201 children with asthma who were not receiving controller therapy. Four guideline-recommended questions (daytime and nocturnal symptoms, school absenteeism, and exercise impairment) determined asthma severity. Concordance between clinician-reported and spirometry-determined asthma severity was poor (kappa = 0.02). Clinical history alone underestimated spirometry-determined disease severity in 27% of children while spirometry results alone underestimated clinician-determined severity in 40% of children.