Abstract: OBJECTIVE To investigate the expressions of peripheral blood microRNA-21 (miR-21) and transforming growth factor-β (TNF-β)/Smad signaling transduction pathway in patients with bronchial asthma complicated with respiratory virus infection. METHODS Totally 109 patients with asthma complicated with respiratory virus infection (study group) and 104 patients without virus infection (control group) in the Third People's Hospital of Gansu Province between Feb.2019 and Feb.2021 were selected for the cross-sectional study.The basic data of the two groups were collected, and parameters including vital signs, lung function, peripheral blood miR-21 and TGF-β/Smad signaling pathway proteins were measured.According to the guidelines, the patients of the two groups were divided into acute exacerbation phase and stable phase.The examination results of each group were compared and the levels of peripheral blood miR-21 and TGF-β/Smad signaling pathway proteins expression of patients with asthma complicated with respiratory virus infection were analyzed. RESULTS In study group, the proportion of respiratory virus infection among 109 patients was 33.94% for influenza virus, 23.85% for human rhinovirus, 19.27% for respiratory syncytial virus, 10.09% for parainfluenza virus, 6.42% for adenovirus, 4.59% for human coronavirus and 1.83% for human metapneumovirus respectively.The proportion of patients with acute exacerbation phase in the study group was higher than that in the control group, and the levels of peripheral blood miR-21, TGF-β1, Smad7, pSmad2 and pSmad3 were higher than those in control group(P＜0.05).The levels of miR-21, TGF-β1, Smad2, Smad3, Smad7, pSmad2 and pSmad3 in peripheral blood of patients with acute exacerbation phase of asthma were higher than those of patients with stable phase of asthma (P＜0.05).There were no statistical differences in peripheral blood miR-21, TGF-β1, Smad2, Smad3, Smad7, pSmad2 and pSmad3 levels in asthma patients with different virus infections. CONCLUSION Early respiratory virus infections might lead to increased expression of peripheral blood miR-21 and increased activation of TGF-β/Smad signaling pathway in patients with asthma, which played an important role in acute attack of asthma.