Allergen extracts for subcutaneous specific immunotherapy (SCIT) may be adsorbed to aluminium hydroxide for adjuvant effect and to delay allergen discharge. A product with an optimised allergen-to-aluminium hydroxide ratio enables a lower allergen dose and shorter introduction phase. The new therapeutic product Avanz (ALK, Denmark) has an optimised allergen-to-aluminium hydroxide ratio. A multicentre, randomised, phase II clinical trial evaluated the immunological effects and tolerability of two different five-step updosing schedules for Avanz in 400 patients with grass pollen-induced rhinoconjunctivitis with or without asthma, randomised into two groups. Group 1 were administered weekly, and group 2 were administered every 3 to 4 days. After updosing, patients received two maintenance injections after 2 and 4 weeks. In the updosing phase, local and systemic reactions were significantly more frequent in group 2 than group 1. In the maintenance phase, adverse reactions were less common in both groups. No generalised reactions with hypotension or fatal reactions occurred. An increase in allergen-specific immunoglobulin G(4) (IgG(4) ) and IgE antibodies was established one week after the last injection. Avanz is a SCIT product with an optimised allergen-to-aluminium hydroxide ratio that enables a shorter updosing phase. We established an improved safety profile with weekly updosing compared with 3-4-day intervals. Clinical trials are needed to confirm clinical efficacy.