Aim:Bronchial hyperresponsiveness(BHR)is a characteristic feature of asthma,but it is also frequently demonstrated by children and adults with chronic obstructive lung diseases.BHR is usually measured by bronchial challenges using direct or indi rect stimuli.The aim of this study was to compare these two types of bronchial challenge in young children with post-infectious bronchiolitis obliterans(BO).Methods:Methacholine and adenosine 5’-monophosphate(AMP)bronchial challenges were performed on preschool children with post-infectious BO(n = 18),those with asthma(n = 23)and in controls(n = 20),using a modified auscultation method.The endpoint was defined as the appearance of wheezing and/or oxygen desaturation.Results:Apositive response to methacholine(an endpoint concentration ≤8 mg/ml)was observed in 88.9%(16/18)of the patients with post-infectious BO,but a positive response to AMP(an endpoint concentration≤200 mg/ml)was observed in only 22.2%(4/18).All patients with asthma responded positively to methacholine,and most(21/23,91.3%)of them also responded positively to AMP.The majority of the controls were insensitive to both challenges.Conclusion:BHR to methacholine is a frequent,but by no means universal,finding in young children with postinfectious BO,but is usually not accompanied by BHR to AMP. Aim: Bronchial hyperresponsiveness (BHR) is a characteristic feature of asthma, but it is also popular demonstrated by children and adults with chronic obstructive lung diseases.BHR is usually measured by bronchial challenges using direct or indi rect stimuli. The aim of this study was To compare these two types of bronchial challenge in young children with post-infectious bronchiolitis obliterans (BO). Methods: Methacholine and adenosine 5’-monophosphate (AMP) bronchial challenges were performed on preschool children with post- infectious BO (n = 18) , those with asthma (n = 23) and in controls (n = 20), using a modified auscultation method. The endpoint was defined as the appearance of wheezing and / or oxygen desaturation. Results: Apositive response to methacholine (an endpoint concentration ≤ 8 mg / ml) was observed in 88.9% (16/18) of the patients with post-infectious BO, but a positive response to AMP (an endpoint concentration ≦ 200 mg / ml) was observed in only 22.2% (4/18 All patients with asthma responde d positively to methacholine, and most (21 / 23,91.3%) of them also responding positively to AMP. majority majority of controls were insensitive to both challenges. Confusion: BHR to methacholine is a frequent, but by no means universal, finding in young children with postinfectious BO, but is usually not to accompanied by BHR to AMP.