Allergen specific immunotherapy has been shown to be effective in rigorous double-blind placebo-controlled clinical trials in both children and adults A recent WHO position paper stated that immunotherapy is an effective treatment for patients with allergic rhinitis/conjunctivitis, allergic asthma and allergic reactions from stinging insects and is thought to be more effective in children than in adults. When speaking about children there are several questions that are important regarding the natural course of the disease. One of the most important is whether immunotherapy can prevent asthma, either by preventing sensitisation to allergens related to the development of asthma or by preventing the inflammation in the lungs caused by allergen exposure. Another point could be to establish the differences in the long term outcome of those patients treated with immunotherapy and medication during childhood, compared to the long term outcome of those with comparable asthma features who received only antiasthmatic medication The PAT study is a European multi-center study. The end-point is to show in what capacity allergen specific subcutaneous immunotherapy can prevent the development of asthma in children who only have rhinoconjunctivitis secondary to grass or birch pollen sensitisation. Two hundred and ten children aged from 5 to 13 years were included in the study. Children were randomised to the active treatment group receiving allergen specific immunotherapy with birch and/or timothy pollen allergen extract or to the control group receiving only pharmacotherapy. It is important to highlight that the main criteria to be included was that the children should never have had any asthmatic symptom. Immunotherapy has been effective in terms of decreasing significantly the amount of symptoms in the active group compared to the control one. It was safe with no serious adverse reactions and reduced the risk of the onset of asthma. After two years of treatment more children in the control group developed clinical asthma than in the active group: p = 0.004. Des Roches et al reported the results of a prospective non randomised trial of immunotherapy with Dermatophagoides pteronyssinus in 44 asthmatic children younger than six years of age who were sensitive only to dust mites. The purpose of the study was to assess whether immunotherapy could reduce the development of new sensitisation during a period of three years of follow up. Specific immunotherapy was given with only D. Pteronyssinus extract. All 22 children in the control group developed new sensitivities as determined by skin testing and in vitro tests, while 10 (45%) of 22 children who received mite immunotherapy did not develop additional sensitivities. The findings of this study suggests that immunotherapy may alter the natural course of the allergic sensitisation reducing the risk of developing new sensitisation in mono sensitive children. A limited number of studies have examined the long terms effects of immunotherapy on the clinical presentation of asthma and rhinoconjunctivitis, and have shown a long lasting efficacy decreasing the amount of symptoms 6-10 years after termination. In a retrospective study of children treated with immunotherapy during childhood for at least three years, that were re-evaluated in early adulthood, the control patients who were treated with medication and no immunotherapy suffered almost 3.5 times more symptoms than the active group treated with immunotherapy. The current findings suggest that immunotherapy should be considered earlier in the course of allergic disease to prevent progression or to prevent the development of new sensitisation. Further studies with long term follow up particularly in children could address this possibility.