Safety and efficacy of sublingual (sublingual-swallow) immunotherapy (IT) with house dust mite extract were evaluated in 30 children (6 - 152/3 years of age) over the first 12 months of an ongoing study. The cumulative dose was 570 μg Der p I (five times that administered with subcutaneous therapy). Safety: One patient on active treatment dropped out after 8 weeks because of a subjective feeling of severe weakness, questionably induced by the therapy. Five patients on active therapy and one patient on placebo reported minor local side effects. Efficacy: Pulmonary symptoms were reduced after 12 months in actively treated asthmatics, but this was not consistent with the lack of improvement in bronchial reactivity, skin sensitivity and specific IgG and IgG4 against D. pt. in this group. In patients with rhinitis nasal sensitivity was reduced in the placebo group without concomitant improvement in the nasal symptom score. Specific IgE (D. pt. and D. f.) in creased significantly more in the active treatment group after 3 and 12 months. We conclude that sublingual IT over 12 months with the fivefold Der p 1 dose of subcutaneous IT was well tolerated, but there was no consistent clinical or immunological benefit compared to placebo.