Objective: Although the precise definition of asthma-chronic obstructive pulmonary disease overlap (ACO) is still controversial, patients sharing common features of both diseases are frequently seen in clinical practice. Current literature suggests that patients with ACO have higher risk of morbidity and mortality than those with asthma or chronic obstructive pulmonary disease (COPD) alone. Omalizumab, a monoclonal anti-IgE monoclonal antibody, has proven to be effective in moderate-to-severe allergic asthma, but data on the efficacy of omalizumab in patients with ACO are limited. To determine the efficacy of omalizumab in patients with ACO. Materials and Methods: We assessed the effectiveness of omalizumab on 12 patients who met the criteria of ACO, using data from medical files of patients with severe allergic asthma who were treated with omalizumab between 2013 and 2018 at a University hospital. Results: Five (41.7%) patients responded well and seven (58.3%) patients responded partially to omalizumab treatment. Decreased number of hospitalizations and exacerbations (p = 0.016 and p = 0.003, respectively) and increased asthma control test results (ACT) (p=0.003) were observed after omalizumab treatment. No significant improvement in pulmonary function tests (FEV1%, FEV1(liter), FEV1/FVC) was found (p=0.444, p=0.208, p=0.510, respectively). Conclusion: Omalizumab was found to reduce asthma exacerbations and improve asthma control in a group of patients with ACO.