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Background: Climate factors and pollen counts may play a role in hay fever., Objective: We sought to determine the impact of specific climate factors and pollen counts on the US prevalence of hay fever and statewide variation in prevalence., Methods: We used a merged analysis of the 2007 National Survey of Children's Health from a representative sample of 91,642 children aged 0 to 17 years and the 2006-2007 National Climate Data Center and Weather Service measurements of relative humidity (%), indoor heating degree days, precipitation, Palmer Hydrological Drought Index, clear sky and issued ultraviolet indices, stratospheric ozone levels, and outdoor air temperature and National Allergy Bureau total pollen counts. Multivariate survey logistic regression models controlled for sex, race/ethnicity, age, household income, and birthplace., Results: The US prevalence of hay fever in childhood was 18.0% (95% CI, 17.7% to 18.2%), with the highest prevalence in southeastern and southern states. Hay fever prevalence was significantly lower with second and third quartile mean annual relative humidity (logistic regression, P ≤ .01 for both), fourth quartile mean annual Palmer Hydrological Drought Index (P = .02), third and fourth quartile mean annual heating degree days (P < .0001 for both), and third and fourth quartile mean annual stratospheric ozone levels but increased with second, third, and fourth quartile mean annual temperature (P ≤ .02 for both), fourth quartile mean annual precipitation (P = .0007), mean total pollen counts (P = .01), and second, third, and fourth quartile issued ultraviolet index (P ≤ .0001 for all). Principal-component analysis was also used to determine the combined effects of correlated climate variables and pollen counts., Conclusions: This study provides evidence of the influence of climate on the US prevalence of childhood hay fever., (Copyright © 2014 American Academy of Allergy, Asthma & Immunology. Published by Elsevier Inc. All rights reserved.)