OBJECTIVES To determine the cost-effectiveness of the grass allergen immunotherapy tablet (AIT), Grazax (ALK-Abelló A/S) for the treatment of grass pollen induced rhinoconjunctivitis in children either with or without co-existing asthma. The analysis compares treatment using Grazax in combination with symptomatic medications, to using symptomatic medications alone in terms of costs and health outcomes. METHODS The model uses a decision tree structure to evaluate two grass pollen induced rhinoconjunctivitis treatments for a cohort of 1000 hypothetical children. The analysis is performed from the perspective of the payer, including only direct costs in the base case. Treatment is modelled in terms of the management of symptoms, reduction in resource use and averting the development of allergic asthma. The evaluation is UK-based and models both short- and long-term effects. Several data inputs, such as resource use and probabilities, are drawn from the paediatric GT-12 clinical trial and the Preventive Allergy Treatment (PAT) study. Cost data are sourced from established published sources and utility values are derived using GT-12 trial data and a published study. RESULTS Treatment with Grazax in combination with symptomatic medications compared to treatment with symptomatic medications alone generates a (discounted) incremental cost per QALY gained of 10,007GBP ($14,578, €11,034), for the base case time horizon of nine years. The QALY gains are a result of increased quality of life related to effective symptom management. The sensitivity analysis carried out around key parameters (e.g. unit costs, medication costs, probabilities, utilities) shows that the results estimated by the model remain robust. CONCLUSIONS Grazax has been shown to improve patient outcomes, at an increased cost. The resulting incremental cost per QALY falls below commonly accepted willingness to pay thresholds. Therefore, Grazax is a cost-effective option for the treatment of grass pollen induced rhinoconjunctivitis in the UK paediatric population.