A follow-up after one year treatment of the previously described active group in a randomized, double-blind, placebo-controlled trial of sublingual immunotherapy (SLIT) for hazelnut food allergy was conducted. The clinical assay was performed from November 2004 to March 2005. Patients in the immunotherapy group were again undergoing treatment from June 2005 to April 2006, all the time with the same daily maintenance schedule, without a build-up phase. The patients were not instructed to avoid or consume hazelnuts from March to June 2005 when patients were not undergoing treatment. At the end of the study, all patients underwent a double-blind placebo-controlled food challenge (DBPCFC) and blood samples were drawn for measurement of specific IgE, IgG4 and serum interleukin 10 (IL-10). The results showed that allergic reactions to hazelnut observed during DBPCFC were generally well-controlled by discontinuing the oral food challenge. No patients required medical treatment. The mean quantity of hazelnut provoking objective symptoms at the beginning was 2.29 g and significantly increased at the end of the first phase of the study to 11.45 g, with almost 50% of patients (5/11) reaching the highest planned dose (20 g). In the second phase of the study, doses significantly increased to 14.57 g and more than 70% of patients (5/7) reached the highest planned dose. However, one patient who had tolerated the highest dose at 3 months lost the immunological protection one year after DBPCFC. Hazelnut-specific IgE levels decreased from 4.23 to 2.36 IU/ml and finally to 1.29 IU/ml. Statistically significant increases in hazelnut-specific IgG4 and IL-10 levels were also observed between the 2 periods. These clinical data confirm significant increases in the threshold of sensitivity to hazelnut allergen to a level that should translate into at least partial protection against most unintended ingestion of hazelnuts.