Background: Guidelines recommend omalizumab in patients with uncontrolled severe allergic asthma. We investigated real-life use of omalizumab, the proportion of patients fulfilling eligibility criteria, its costs and its effectiveness. Method: In a cohort of asthma patients initiating treatment with omalizumab in Belgium between 2010 and 2016, we investigated fulfilment of eligibility criteria (chronic use of high-dose inhaled corticosteroids (ICSs) plus long-acting beta(2)-agonists (LABAs) and >= 2 severe asthma exacerbations in previous year), and compared hospitalisations and systemic corticosteroid consumption in the year before and after omalizumab initiation. We computed healthcare costs in the respective time periods and compared the cost per prevented hospitalisation in patients fulfilling eligibility criteria versus those who did not. Results: Between 2010 and 2016, omalizumab treatment was initiated in 2068 patients with asthma; only 24% fulfilled the eligibility criteria, mainly due to nonadherence to high-dose ICSs + LABAs. The proportion of patients hospitalised for asthma decreased from 41% to 21% in eligible patients (absolute risk reduction, 20%), whereas the absolute risk reduction was 5% ( from 19% to 14%) in noneligible patients. The cost per prevented hospitalisation was (sic)44238 versus (sic)139495, respectively. Chronic use of systemic corticosteroids was discontinued in 35% of eligible patients versus 15% of noneligible patients. Conclusion: In Belgium, omalizumab is mostly initiated in uncontrolled asthma patients who are nonadherent to ICSs + LABAs. Omalizumab decreases hospitalisations and the use of systemic corticosteroids, but at a high cost. Careful management of patients with difficult-to-treat asthma should be a priority before prescribing omalizumab.