BACKGROUND: Immunotherapy with the SQ-standardized grass tablet Grazax is efficacious and well-tolerated in adult patients with rhinoconjunctivitis. Allergic asthma and rhinoconjunctivitis are closely linked, and a strategy combining treatment of the upper and lower airways is recommended. OBJECTIVE: To investigate the efficacy of treatment with the grass tablet on grass pollen-induced rhinoconjunctivitis and asthma as well as the immunologic response and the safety profile in children. METHODS: A total of 253 children age 5 to 16 years, with grass pollen-induced rhinoconjunctivitis with/without asthma, were randomized 1:1 to active treatment or placebo. Treatment was initiated 8 to 23 weeks before the start of the grass pollen season 2007 and continued throughout the entire season. Symptomatic medication was provided as relief medication to both groups in a stepwise fashion. Primary endpoints were rhinoconjunctivitis symptom and medication scores. RESULTS: The rhinoconjunctivitis symptom and medication scores and the asthma symptom score were all statistically significantly different between the 2 treatment groups. The differences in medians relative to placebo were 24%, 34%, and 64% in favor of active treatment. The immunologic response was similar to that observed in adults. The most common adverse reaction was oral pruritus, reported by 40 subjects (32%) in the active and 3 (2%) in the placebo group. Six subjects withdrew because of adverse events. No serious adverse events were assessed as treatment-related. CONCLUSION: Immunotherapy with the grass tablet reduced grass pollen-induced rhinoconjunctivitis and asthma symptoms in a pediatric population and introduced an immunomodulatory response, consistent with treatment of the underlying allergic disease. The treatment was well tolerated.