Food allergy is generally thought to be lifelong, there might be a substantial number of adult patients labelled as having food allergy who have in fact outgrown their allergy (1). On the other hand, interleukin-10 (IL10) is a major regulatory cytokine of inflammatory responses which plays a key role in the induction and maintenance of anergic states (2). Moreover, T-cell response to allergens entering the mucosal route is actively suppressed by IL-10 in healthy subjects (3). The aim of this study was to assess the usefulness of serum IL10 levels for identifying oral tolerance in patients suffering from hazelnut food allergy as assessed by double-blind, placebo-controlled food challenge (DBPCFC). This was a prospective study. Patients provided written informed consent and the study was approved by the ethics committees of the participating hospitals. Forty-one adult patients aged 18–60 were selected on the basis of a clear history of hazelnut food allergy and positive skin prick test (SPT) to hazelnut. Doubleblind, placebo-controlled food challenge were performed in all patients, based on a previous European multicentre study (4), and were positive in 29 (70.7%) and negative in 12 (29.3%). After DBPCFC to hazelnut, patients were divided into two groups according to the food challenge results (positive or negative). No statistical differences were found between patients with DBPCFC-positive results and DBPCFC-negative results to the oral food challenges for age, hazelnut allergy symptoms and time of the last hazelnut food allergic reaction (Table 1). However, differences in sex distribution was found between both groups, with a lower number of female patients in the DBPCFC-negative group. No statistical differences were also detected by the parallel line assay for raw hazelnut-SPT performed in both groups. Hazelnut-specific IgE and IgG4 were measured (Hytec-specific IgE and IgG4 enzyme immuno-assay; Hycor Biomedical Inc., Garden Grove, CA, USA). Human serum IL-10 was also measured by enzyme-linked immunosorbent assay (Biolink 2000, Barcelona, Spain). Hazelnut-specific IgE levels did not differ between groups. Regarding hazelnut-specific IgG4 levels, no statistical significance was found between groups, although high IgG4 mean values were observed in the hazelnut-tolerant patients. High IL-10 mean values were also found in the DBPCFC-negative group with statistical significance (P < 0.03) (Fig. 1). The receiver operating curve which established the cutoff offering the best sensitivity and specificity for identifying hazelnut tolerance was chosen to study the usefulness of serum IL-10 levels measurement. Serum IL-10 levels 2.28 pg/ml are considered capable of identifying hazelnut tolerance with sensitivity of 82% and specificity of 70% (CI: 68–97%). Although the DBPCFC is the gold standard for the diagnosis of food allergy, increasingly higher concentrations of food-specific IgE correlate with an increasing likelihood of a clinical reaction and may help to avoid oral food challenges in some cases (5). Interleukin10 cooperates in the regulatory T-cell response to allergens in healthy subjects, furthermore, high IL-10 levels during and after specific immunotherapy aid tolerance to these allergens (3, 6). Thus, serum IL-10 levels measurement could be useful in the determination of food tolerance in previously food-allergic patients. In the current study, high IL-10 mean values were found in the DBPCFC-negative group with statistical significance, suggesting a role for this cytokine in the outgrowing of food allergy. Moreover, serum IL-10 levels 2.28 pg/ml are considered capable of identifying hazelnut tolerance with sensitivity of 82% and specificity of 70%, with such sensitivity and specificity, the dosage of IL-10 is nowadays unable to perform as diagnostic or screening test for food tolerance, but the data open room to wider studies in different settings in order to explore the possibility to use it as an additional diagnostic tool in food allergy. This high IL-10 level obtained in the DBPCFC-negative group suggests a role for this cytokine in outgrowing food allergy, rendering this interleukin a useful The increase in Interleukin-10 levels in the DBPCFC-negative group suggests a role for this cytokine as useful tool in the diagnosis of food tolerance in previously foodallergic patients.