OBJECTIVE Increases in asthma and obesity over the past three decades have led to speculation about a causal link between the two diseases. However, investigations of the relationship between body mass index (BMI) and fractional exhaled nitric oxide (FeNO) - a marker of eosinophilic airway inflammation - have produced mixed results. The purpose of this study was to evaluate the relationship between body mass index (BMI), asthma and FeNO in a sample of U.S. adults using data from the National Health and Nutrition Examination Surveys (NHANES) for 2007-2010. METHODS We assessed the relationship between FeNO and BMI in subjects with and without asthma using categorical and continuous models for BMI. All models controlled for age, gender, ethnicity, household income-to-poverty ratio, atopy and current smoking. RESULTS Adjusted asthma prevalence was positively associated with BMI, and subjects with asthma had higher adjusted FeNO levels than subjects without asthma. However, no association between FeNO and BMI was observed in either those with (β = 0.002, p = 0.74) or without (β = 0.0014, p = 0.51) asthma after adjusting for covariates. CONCLUSIONS Our results suggest that in the U.S. adult population, BMI is not associated with eosinophilic airway inflammation.