Background The treatment of allergic asthma by specific immunotherapy (SIT) is hampered by potential side‐effects.

Objective The aim of this study was to study the effect of omalizumab, a monoclonal anti‐IgE antibody, in combination with SIT in patients with seasonal allergic rhinoconjunctivitis (SAR) and co‐morbid seasonal allergic asthma (SAA) incompletely controlled by conventional pharmacotherapy.

Methods A randomized, double‐blind, placebo‐controlled, multi‐centre trial was performed to assess the efficacy and safety of omalizumab (Xolair^®) vs. placebo in combination with depigmented SIT (Depigoid^®) during the grass pollen season. Omalizumab or placebo was started 2 weeks before SIT; the whole treatment lasted 18 weeks. Primary endpoint was daily ‘symptom load’, the sum of daily scores for symptom severity and rescue medication use.

Results A total of 140 patients (age 11–46 years) were randomized; and a total of 130 finished the study. Combination therapy reduced the symptom load by 39% (P=0.0464, Wilcoxon test) over SIT monotherapy. This difference was mainly due to reduced symptom severity (P=0.0044), while rescue medication use did not change significantly. Combination therapy also improved asthma control (Asthma Control Questionnaire, P=0.0295) and quality of life in the case of asthma (Asthma Quality of Life Questionnaire, P=0.0293) and rhinoconjunctivitis (Rhinoconjunctivitis Quality of Life Questionnaire, P=0.0537). Numbers of patients with ‘excellent or good’ treatment efficacy according to ratings of investigators (75.0% vs. 36.9%) or patients (78.5% vs. 46.1%) were markedly higher in the combination group than under SIT alone.

Conclusion Combination of omalizumab with SIT for treatment of patients with SAR and co‐morbid SAA was safe and reduced the symptom load in a statistically significant and clinically meaningful manner.