Rationale: Alveolar nitric oxide concentration (F_ANO), increasingly considered in asthma, is currently interpreted as a reflection of nitric oxide (NO) production in the alveoli. Recent modelling studies showed that axial molecular diffusion brings NO molecules from the airways back into the alveolar compartment during exhalation ("back-diffusion") and contributes to F_ANO. Objective: 1) to simulate the impact of back-diffusion on F_ANO and to estimate the alveolar concentration actually due to in-situ production (F_ANO_prod). 2) To determine actual alveolar production in stable asthma patients with a broad range of NO bronchial productions. Methods: 1) A model incorporating convection and diffusion transport and NO sources was used to simulate F_ANO and exhaled NO concentration at 50 ml.s^-1 expired flow (F_ENO) for a range of alveolar and bronchial NO productions. 2) F_ANO and F_ENO were measured in 10 healthy subjects (8 males; age: 38 ± 14 years) and in 21 asthma patients with stable asthma (16 males; age: 33 ± 13 years, FEV₁ : 98.0 ± 11.9%pred ). The Asthma Control Questionnaire of Juniper assessed asthma control. Results: Simulations predict that, due to back-diffusion, F_ANO and F_ENO are linearly related. Experimental results confirm this relationship. F_ANO_prod may be derived by F_ANO_prod = (F_ANO-0.08·F_ENO)/0.92 (Equation 1). Based on (1), F_ANO_prod is similar in asthma patients and in healthy subjects. Conclusion: The back-diffusion mechanism is an important determinant of NO alveolar concentration. In stable and unobstructed asthma patients, even with increased bronchial NO production, alveolar production is normal when appropriately corrected for back-diffusion.