Background: The relationship among inhaled allergen exposure, sensitization, and asthma severity is unknown. Objectives: To investigate the relationship among personal allergen exposure, reservoir dust allergen concentrations, and physiological measures of asthma severity; to examine the numbers of particles inspired that react with autologous IgE and IgG 4. Methods: A total of 117 patients with asthma wore 5 nasal air samplers (NASs) at home: 1 each for exposure to mite, cat and dog allergens, NAS-IgE, and NAS-IgG4. NASs were processed by HALOgen assay for allergen measurement and incubated with autologous serum for detection of NAS-IgE and NAS-IgG4. Reservoir allergen concentrations were measured by ELISA. Subjects' asthma severity was ascertained by measurement of lung function, exhaled nitric oxide, and nonspecific bronchial reactivity to histamine. Results: Nasal air sampler counts correlated with reservoir concentrations for cat (r = 0.31; P = .001) and dog (r = 0.20; P = .03) but not mite allergen (r = 0.001; P = 1.0). There was no significant relationship between sensitization with exposure measured by NAS to any allergen and PD 20FEV1 (F[3,60] = 1.60; P = .20); however, sensitization with exposure in dust reservoirs had significant effects on PD 20FEV1 for any allergen (F[3,59] = 3.12; P = .03), cat (F[3,59] = 3.77; P = .01), and mite (F[3,59] = 2.78; P = .05), but not dog (F[3,59] = 1.06; P = .37). We repeated the analysis with separate variables for sensitization and exposure, controlling for the confounders; sensitization but not exposure conferred lower PD20FEV1 values. However, increasing cat allergen exposure was associated with improving bronchial reactivity in not cat-sensitized patients. NAS-IgE and NAS-IgG4 counts bore no relationship to any measure of asthma severity. Conclusion: Nasal air samplers confer no advantage over reservoir dust analysis for studies of asthma severity. In common with other measures of exposure, single nasal air samples do not provide a useful measure of home allergen exposure for the individual patient with allergic asthma. © 2006 American Academy of Allergy, Asthma and Immunology.