Background: Sublingual allergen immunotherapy (SLIT) has been demonstrated to be both clinically efficacious and safe. However, in line with the current regulatory guidance from the European Medicines Agency, allergen immunotherapy (AIT) products must demonstrate their efficacy and safety in pivotal phase III trials for registration. Objective: We sought to investigate the efficacy and safety of sublingual high-dose liquid birch pollen extract (40,000 allergy units native [AUN]/mL) in adults with birch pollen allergy. Methods: A randomized, double-blind, placebo-controlled, parallel-group multicenter trial was conducted in 406 adult patients with moderate-to-severe birch pollen-induced allergic rhinoconjunctivitis with or without mild-to-moderate controlled asthma. Treatment was started 3 to 6 months before the birch pollen season and continued during the season in 40 clinical study centers in 5 European countries. For primary end point assessment, the recommended combined symptom and medication score of the European Academy of Allergy and Clinical Immunology was used. Secondary end points included quality-of-life assessments, immunologic parameters, and safety.Results: Primary efficacy results demonstrated a significant (P < .0001) and clinically relevant (32%) reduction in the combined symptom and medication score compared with placebo after 3 to 6 months of SLIT. Significantly better rhinoconjunctivitis quality-of-life scores (P < .0001) and the patient's own overall assessment of his or her health status, including the visual analog scale score (Euro Quality of Life Visual Analogue Scale; P 5 .0025), were also demonstrated. In total, a good safety profile of SLIT was observed. Conclusion: This study confirmed both the clinical efficacy and safety of a sublingual liquid birch pollen extract in adults with birch pollen allergy in a pivotal phase III trial (EudraCT: ; g ALIAN s.r.o. Ambulancia alergol ogie a klinickej imunol ogie, Bardejov; and h HAL Allergy B.V., Leiden. The results of this study were presented in part as poster presentations at annual congresses of the European Academy of Allergy and Clinical Immunology (EAACI) in Vienna, Austria, June 2016. Sponsored by HAL Allergy B.V., Leiden, The Netherlands. The sponsor also paid for open access of the article (online open publication). Disclosure of potential conflict of interest: O. Pfaar reports grants and personal fees from HAL Allergy B.V. during the conduct of the study; grants and personal fees from ALK-Abell o, Allergopharma, Stallergenes Greer, HAL Allergy BA.; grants and personal fees from Laboratorios LETI/LETI Pharma; personal fees from Novartis Pharma and MEDA Pharma; grants and personal fees from Anergis S.A.; personal fees from Mobile Chamber Experts (a GA 2 LEN Partner), Pohl-Boskamp, and Indoor Biotechnologies; and grants from GlaxoSmithKline all outside the submitted work. C. Bachert reports grants and personal fees from HAL, ALK-Abell o, Stallergenes Greer, Biotech Tools, Novartis Pharma, Mylan, GlaxoSmithKline, Sanofi Aventis, and Ast...