BACKGROUND: Cedar pollinosis (CP), a common form of seasonal allergic rhinitis (AR), is a substantial medical problem in Japan due to its high prevalence and severe symptoms. Omalizumab (anti-IgE therapy) has previously proven to be effective in CP/AR, but no studies for inadequately controlled severe CP/AR despite standard-of-care (SoC) have been conducted. OBJECTIVE: To determine the efficacy of omalizumab added to SoC in patients with inadequately controlled severe CP in a randomized, double-blinded, placebo-controlled, phase III study. METHODS: Adult/adolescent patients with severe CP whose symptoms were inadequately controlled despite nasal corticosteroids plus 1 or more oral medications in the previous 2 seasons were randomized to receive omalizumab (n = 162) or placebo (n = 175). All patients received concomitant antihistamines and nasal corticosteroids as SoC. The primary endpoint was the mean nasal symptom score during the severe symptom period. Secondary endpoints included mean ocular symptom score, quality of life (QoL), and safety. RESULTS: The SoC + omalizumab treatment had statistically significantly and clinically important lower nasal (least squares mean difference, -1.03, P < .001) and ocular (-0.87, P < .001) symptom scores compared with SoC + placebo, respectively. Differences in scores for individual components of nasal and ocular symptoms were also statistically and clinically significant. SoC + omalizumab also improved QoL scores as overall and in all domains. No unexpected safety signals were observed. CONCLUSIONS: In patients with severe CP, omalizumab added to SoC demonstrated consistent efficacy in improving symptoms and QoL, and was well tolerated. These results indicate that omalizumab could be a promising therapeutic option for severe CP/AR.