Mastocytosis is a clonal disorder characterized by the proliferation and accumulation of mast cells (MC) in different tissues, with a preferential localization in skin and bone marrow (BM). The excess of MC in mastocytosis as well as the increased releasability of MC may lead to a higher frequency and severity of immediate hypersensitivity reactions. Mastocytosis in adults is associated with a history of anaphylaxis in 22-49%. Fatal anaphylaxis has been described particularly following hymenoptera stings, but also occasionally after the intake of drugs such as nonsteroidal anti-inflammatory drugs, opioids and drugs in the perioperative setting. However, data on the frequency of drug hypersensitivity in mastocytosis and vice versa are scarce and evidence for an association appears to be limited. Nevertheless, clonal MC disorders should be ruled out in cases of severe anaphylaxis: basal serum tryptase determination, physical examination for cutaneous mastocytosis lesions, and clinical characteristics of anaphylactic reaction might be useful for differential diagnosis. In this position paper, the ENDA group performed a literature search on immediate drug hypersensitivity reactions in clonal MC disorders using MEDLINE, EMBASE, and Cochrane Library, reviewed and evaluated the literature in five languages using the GRADE system for quality of evidence and strength of recommendation.Mastocytosis is a clonal disorder characterized by the proliferation and accumulation of mast cells (MC) in different tissues, with preferential localization in skin and bone marrow (BM). The great majority of cases are associated with somatic gainof-function point mutations of KIT, a tyrosine kinase receptor (CD117). The clinical presentation of mastocytosis is heterogeneous ranging from symptoms limited to the skin (with frequent spontaneous regression in pediatric cases) to a more aggressive systemic variant (SM), typical only of adulthood. Mastocytosis patients can experience symptoms due to massive MC activation and release of mediators (e.g., generalized pruritus, urticaria/angioedema, abdominal pain, anaphylaxis), but the presence of mediator symptoms does not correlate with the allele burden of the KIT mutation (1).The current classification of mastocytosis was developed in 2000 (2) and accepted by the World Health Organization (WHO) in 2001 (3) and then updated in 2008 (4) ( Table 1). According to the WHO classification, two major categories are defined: cutaneous mastocytosis (CM), limited to the skin, and systemic mastocytosis (SM), involving extracutaneous organs, mainly BM (Table 1) (4). The diagnosis of SM requires the presence of one major criterion and one minor criterion, or alternatively three minor criteria (Table 2) Systemic variant is suspected in patients with mastocytosis in the skin (MIS) and also in patients without MIS who have systemic symptoms of MC mediator release, including anaphylaxis. Patients without MIS may be a diagnostic challenge. The European Competence Network on Mastocytosis (ECNM) recently ...