OBJECTIVES: : To assess, in real life conditions, the costs, effects, and cost-effectiveness of Xolair® (omalizumab) as add-on versus conventional therapy in the management of Belgian patients with severe persistent allergic asthma inadequately controlled.  METHODS: : The same Markov model used for omalizumab initial reimbursement dossier was populated with data from a Belgian observational study, i.e. PERSIST study (n=160), especially set up to address questions raised by the Belgian authorities for omalizumab re-evaluation.  The model takes into account four health states and links effectiveness data, real life resource use, and utility data.  Medical resources use (drug treatment, laboratory tests and procedures, physician consultations, emergency room visits, and hospitalisations) were collected in PERSIST and costed from the perspective of the health care payer (i.e. INAMI/RIZIV + patient).  EQ-5D data were also collected during the study and used in the Markov model.  RESULTS: : Over a lifelong time horizon, the expected average numbers of life years (LYs) and QALYs per patient, for conventional therapy are 18.33 and 9.80, respectively.  For omalizumab, the respective figures are 22.19 and 12.54.  Over a lifelong time horizon, the expected average costs per patient are €44,548 and €124,726 for conventional therapy and omalizumab as add-on, respectively.  Hence, omalizumab ICERs are €20,777/LY gained and €29,187/QALY gained.  This compares very favourably (cf. substantial decrease in the ICERs) with the initial submission ICERs (i.e. €40,370/LY gained and €42,669/QALY gained).  The sensitivity analyses performed show that results can somewhat vary according to the parameters changed.  However, in all cases, the ICERs obtained with real life data are always markedly inferior to what was calculated in our initial submission.  CONCLUSIONS: The pharmacoeconomic evaluation based on data from a real life Belgian study confirms that omalizumab is cost-effective versus conventional therapy in the treatment of patients with severe persistent allergic asthma inadequately controlled.