Omalizumab is a monoclonal anti-IgE antibody used to treat severe allergic asthma (SAA). The aim of the STELLAIR study was to determine the importance of pre-treatment blood eosinophil count as a predictive measure for response to omalizumab.This retrospective real-life study was conducted in France between December 2015 and September 2016 using medical records of SAA omalizumab-treated patients. Response to omalizumab was assessed by three criteria: physician evaluation, reduction of >/=40% in annual exacerbation rate and a combination of both. Response rate was calculated according to blood eosinophil count measured in the year prior to omalizumab initiation.872 SAA omalizumab-treated patients were included by 78 physicians (723 adults (age >/=18 years) and 149 minors (age 6-17 years)). Blood eosinophil count was >/=300 cells.microL(-1) in 52.1% of adults and 73.8% of minors. By physician evaluation, 67.2% of adults and 77.2% of minors were responders and 71.1% adults and 78.5% minors had a >/=40% reduction in the exacerbation rate. In adults, the response rate for combined criteria was 58.4% (95% CI 53.2-63.4%) for blood eosinophils >/=300 cells.microL(-1) (n=377) and 58.1% (95% CI 52.7-63.4%) for blood eosinophils <300 cells.microL(-1) (n=346).This study shows that a large proportion of patients with SAA have a blood eosinophil count >/=300 cells.microL(-1), and suggests that omalizumab effectiveness is similar in "high" and "low" eosinophil subgroups.Read less <