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Background Fractional exhaled nitric oxide (FeNO) has a potential clinical role in asthma management. Constitutive factors such as age, height and gender, as well as individual characteristics, such as IgE sensitization and smoking, affect the levels of FeNO in population-based studies. However, their effect on FeNO in subjects with asthma has been scarcely studied. Objective To study the effects on FeNO of these commonly regarded determinants, as demonstrated in healthy subjects, as well as menarche age and parental smoking, in a population of asthmatics. Material and Methods Fractional exhaled nitric oxide was measured in 557 subjects with asthma from the Swedish GA2 LEN study. Allergic sensitization was assessed by skin prick tests to most common aeroallergens. Upper airway comorbidities, smoking habits, smoking exposure during childhood and hormonal status (for women) were questionnaire-assessed. Results Male gender ( P < 0.001), greater height ( P < 0.001) and sensitization to both perennial allergens and pollen ( P < 0.001) are related to higher FeNO levels. Current smoking ( P < 0.001) and having both parents smoking during childhood, vs. having neither ( P < 0.001) or only one parent smoking ( P = 0.002), are related to lower FeNO. Women with menarche between 9 and 11 years of age had lower FeNO than those with menarche between 12 and 14 years of age ( P = 0.03) or 15 and 17 years of age ( P = 0.003). Conclusions and Clinical relevance Interpreting FeNO levels in clinical practice is complex, and constitutional determinants, as well as smoking and IgE sensitisation, are of importance in asthmatic subjects and should be accounted for when interpreting FeNO levels. Furthermore, menarche age and parental smoking during childhood and their effects on lowering FeNO deserve further studies. [ABSTRACT FROM AUTHOR]