Omalizumab, a recombinant humanized monoclonal antibody, is the first therapeutic agent specifically targeting immunoglobulin E (IgE). It has been investigated extensively in the treatment of patients with allergic diseases and is approved in the United States for the treatment of patients with moderate-to-severe persistent asthma. In this setting, omalizumab reduced the frequency and incidence of asthma exacerbations and asthma control was well maintained, even though patients significantly reduced their dose of inhaled corticosteroid. Importantly, omalizumab has been shown to reduce asthma exacerbations in high-risk patients (eg, with previous intubation and hospitalization) and to reduce the level of severe asthma exacerbations (those resulting in emergency treatment and hospitalization). Responder analysis shows that omalizumab provides the greatest benefit in patients with more severe asthma, and this has been confirmed by recent studies establishing the efficacy of omalizumab in patients whose asthma is poorly controlled despite receiving the best standard care in medication. Omalizumab also has proved to be effective in patients with seasonal and perennial allergic rhinitis and to improve quality of life for patients with asthma or rhinitis. As expected with a systemic anti-IgE agent, omalizumab was shown to be effective in the treatment of patients with concomitant asthma and allergic rhinitis. The efficacy of omalizumab in a range of allergic diseases reaffirms the importance of IgE in the pathogenesis of these conditions and establishes the potential benefit to be obtained by inhibiting IgE, especially in patients with more severe and comorbid allergic diseases.