Dysfunction of small airway in severe asthmatic patients could be useful in assessing the response to biologics. Objective:To analyse small airway in uncontrolled severe asthmatics before starting anti-IL5 receptor therapy and to evaluate its changes with treatment. Methods:Observational prospective study of patients starting treatment with Benralizumab in our Asthma Unit since July 2021 with a minimum follow-up of 6 months. All patients maintained a treatment with high-dose GCI/LABA/LAMA that has not been changed with the addition of Benralizumab. Both FENO, spirometry, impulse oscillometry, blood eosinophilia and questionnaires were performed at the start of the biologic and at 6 months. Significance level p<0.05. Results:12 patients were included. 59% women, mean age 62+11 years. 33% of the patients associated nasal polyposis. FEV1 (% predicted):76%+18; MMEF (% predicted):39.50% +25.52; FENO:55+30 ppb. Eosinophils 591+277cell/mcL. Impulse oscillometry showed that the mean total airway resistance (R5) decreased after 6 months of treatment (from 5.15 to 4.22 kPa/l/s; p=0.021), as well as that of the peripheral resistances(from 1.34 to 0.89 kPa/l/s; p=0.047), reactance area(from 38.68 to 21.08 kPa/l; p=0.060) and resonance frequency(23.50 to 18.66 kPa /l/s; p=0.011). There was also a significant improvement in peripheral eosinophilia, FeNO, FEV1, MMEF, asthma control and the EVAS global health scale, with a reduction in exacerbations. Conclusion:the addition of Benralizumab to previous treatment produces an improvement in small airways measured by oscillometry at 6 months as well as in other variables that are usually used to measure response to biologics.