Aim: The protective effect of allergen immunotherapy against a new allergic sensitization iscontroversial. This study aimed to investigate the effect of allergen immunotherapy on newallergic sensitization in children.Materials and Methods: The study included 50 patients who received immunotherapy for atleast 3 years, and whose skin prick tests were repeated at intervals of at least 3 years (31 patientsfor house dust mite immunotherapy, 19 patients for pollen immunotherapy), and 69 controlswith similar characteristics.Results: The number of patients who developed a new sensitization was similar both in thegroups of patients who received house dust mite and pollen immunotherapy, and the controlgroup. There was no significant difference between the first and last skin prick tests ofthe patients who received house dust mite and pollen immunotherapy; however, in the controlgroups, a significant increase in sensitivity to tree pollens (n = 2, 5.4%; n = 8, 21.6%) and weedpollens (n = 7, 26.9%; n = 14, 53.8%) was detected (P = .031 and P = .039). While allergen sensitivitiesin the first tests of the pollen immunotherapy group and the control group were similar,weed pollen sensitivity was significantly higher in the last tests of the control group (n = 14,53.8%; n = 4, 21.1%, P = .027). It was determined that the presence of weed pollen sensitization(OR: 8.1, 95% CI: 1.5-42.4) and having asthma (OR: 3.5, 95% CI: 1.3-10.8) increases the risk of newsensitization in all groups.Conclusion: Allergen immunotherapy has been found to protect against new sensitizationto tree and weed pollens. However, this effect was insignificant in the multivariate analysis.Weed pollen sensitization and the presence of asthma are related to the development of newsensitization.