IgE treatment with omalizumab was shown to be effective in chronic 7 spontaneous urticaria (CSU) in randomized controlled trials (1) and real life studies 8 (2) (3). Omalizumab, the first licensed therapy for H 1-antihistamine refractory CSU, 9 prevents IgE binding to its high-affinity receptor (FcεRI) (4). As a result, free serum 10 IgE levels decline and the expression of FcεRI receptors on basophils and mast cells 11 is down-regulated (4). Moreover, omalizumab appears to dissociate pre-bound IgE 12 from mast cells and basophils (4). 13 The exact mechanisms of action of omalizumab in CSU are not fully understood, and 14 the therapeutic response remains difficult to predict. The characterization of 15 responders vs. non-responders to omalizumab therapy could help to understand its 16 mode of action and to establish predictors of response in clinical practice. 17 Here, we compared complete responders (CR) to non-responders (NR) and partial 18 responders (PR) to identify predictors for response. In total, we analyzed 85 patients 19 from 4 GA²LEN urticaria centers of reference and excellence (UCAREs) (5), 43 CR, 20 23 PR, and 19 NR to omalizumab treatment (Table 1). Complete, partial and non-21 response was defined as the reduction of signs and symptoms by ≥90%, by ≥30% 22 but <90%, and by <30% (physicians global assessment), respectively, after 2 four-23 weekly injections of omalizumab 300 mg. All centers provided data on NRs 24 comparing patient characteristics, laboratory values, and treatment response. In 25 addition, all centers were asked to provide data on at least one PR and CR per 26 entered NR. Because different methods of total IgE assessment with different 27 reference values were used, the total IgE-values were transferred to percentage 28 scores of the respective upper reference value. Total IgE levels of 0%-10%, >10%-29 50%, and >50%-100% were regarded as low normal, mid normal, and high normal, 30 whereas total IgE levels of >100%-250%, >250%-1000%, and >1000% were 31 regarded as slightly elevated, moderately elevated, and highly elevated. Statistical 32 analyses were done with SPSS (IBM SPSS Statistics, Version 22).