ASTERIA I was a 40-week, randomized, double-blind, placebo-controlled study to evaluate the efficacy and safety of subcutaneous omalizumab as add-on therapy for 24 weeks in patients with chronic idiopathic urticaria/spontaneous urticaria (CIU/CSU) who remained symptomatic despite H-1 antihistamine treatment at licensed doses. Patients aged 12-75 years with CIU/CSU who remained symptomatic despite treatment with approved doses of H-1 antihistamines were randomized (1:1:1:1) in a double-blind manner to subcutaneous omalizumab 75 mg, 150 mg, or 300 mg or placebo every 4 weeks for 24 weeks followed by 16 weeks of follow-up. The primary end point was change from baseline in weekly itch severity score (ISS) at week 12. Among randomized patients (N=319: placebo n=80, omalizumab 75 mg n=78, 150 mg n=80, 300 mg n=81), 262 (82.1%) completed the study. Compared with placebo (n=80), mean weekly ISS was reduced from baseline to week 12 by an additional 2.96 points (95% confidence interval (Cl): -4.71 to -1.21; P=0.0010), 2.95 points (95% CI: -4.72 to -1.18; P=0.0012), and 5.80 points (95% Cl: -7.49 to -4.10; P<0.0001) in the omalizumab 75-mg (n=77), 150-mg (n=80), and 300-mg groups (n=81), respectively. The omalizumab 300-mg group met all nine secondary end points, including a significant decrease in the duration of time to reach minimally important difference response (>= 5-point decrease) in weekly ISS (P<0.0001) and higher percentages of patients with well-controlled symptoms (urticaria activity score over 7 days (UAS7) <= 6: 51.9% vs. 11.3%; P<0.0001) and complete response (UAS7 = 0: 35.8% vs. 8.8%; P<0.0001) versus placebo. During the 24-week treatment period, 2 (2.9%), 3 (3.4%), 0, and 4 (5.0%) patients in the omalizumab 75-mg, 150-mg, 300-mg, and placebo groups, respectively, experienced a serious adverse event. Omalizumab 300 mg administered subcutaneously every 4 weeks reduced weekly ISS and other symptom scores versus placebo in CIU/CSU patients who remained symptomatic despite treatment with approved doses of H-1 antihistamines.