BACKGROUND: Many patients with chronic idiopathic urticaria (also called chronic spontaneous urticaria) do not have a response to therapy with H 1 -antihistamines, even at high doses. In phase 2 trials, omalizumab, an IgE monoclonal antibody that targets IgE and affects mast-cell and basophil function, has shown efficacy in such patients. METHODS: In this phase 3, multicenter, randomized, double-blind study, we evaluated the efficacy and safety of omalizumab in patients with moderate-to-severe chronic idiopathic urticaria who remained symptomatic despite H 1 -antihistamine therapy (licensed doses). We randomly assigned 323 patients to receive three subcutaneous injections, spaced 4 weeks apart, of omalizumab at doses of 75 mg, 150 mg, or 300 mg or placebo, followed by a 16-week observation period. The primary efficacy outcome was the change from baseline in a weekly itch-severity score (ranging from 0 to 21, with higher scores indicating more severe itching). RESULTS: The baseline weekly itch-severity score was approximately 14 in all four study groups. At week 12, the mean (±SD) change from baseline in the weekly itch-severity score was -5.1±5.6 in the placebo group, -5.9±6.5 in the 75-mg group (P = 0.46), -8.1±6.4 in the 150-mg group (P = 0.001), and -9.8±6.0 in the 300-mg group (P<0.001). Most prespecified secondary outcomes at week 12 showed similar dose-dependent effects. The frequency of adverse events was similar across groups. The frequency of serious adverse events was low, although the rate was higher in the 300-mg group (6%) than in the placebo group (3%) or in either the 75-mg or 150-mg group (1% for each). CONCLUSIONS: Omalizumab diminished clinical symptoms and signs of chronic idiopathic urticaria in patients who had remained symptomatic despite the use of approved doses of H 1 -antihistamines. (Funded by Genentech and Novartis Pharma; ClinicalTrials.gov number, NCT01292473.) Copyright © 2013 Massachusetts Medical Society.