Specific immunotherapy is a well-established treatment for allergic rhinoconjunctivitis; conventional regimens are lengthy, however, reducing convenience and cost-effectiveness. This study evaluated the efficacy and safety of an ultrashort course (four doses) of the immunotherapy Grass Modified Allergen Tyrosine Adsorbate (Allergy Therapeutics, Worthing, U.K.) monophosphoryl lipid A (MATA MPL). Subjects were randomized to receive four injections of either Grass MATA MPL (n = 514; 300-2000 standardized units/injection) or placebo (n = 514) before the grass pollen season. They used electronic diaries to record allergy symptoms and medication use during the pollen season. The primary end point was the difference between the mean combined symptom and medication scores in the Grass MATA MPL and placebo groups during the 4 local peak pollen weeks. The injection course was completed by 95.3 and 97.7% of the Grass MATA MPL and placebo groups, respectively, and was well tolerated. Grass MATA MPL treatment afforded a 13.4% benefit over placebo in the 4 peak pollen weeks (p = 0.0038). The benefit in subjects with 28 complete diary entries during the 4 peak pollen weeks was 26.9% (p = 0.0031). Significant benefits over placebo were observed in subjects with severe symptoms (17.1%; p = 0.0023), in those who had a history of allergic rhinoconjunctivitis for up to 35 years (up to 37.2%; p = 0.0059) and at sites with a higher burden of disease (38.3%; p < 0.0001). The ultrashort course of Grass MATA MPL was well tolerated and provided a significant benefit over placebo in relieving allergy symptoms.