BackgroundFood protein-induced enterocolitis syndrome (FPIES) is a delayed, non-IgE-mediated food allergy. This syndrome was once thought to be rare, but emerging literature suggests an increasing incidence along with more foods being implicated. Also, with the introduction of guidelines on early peanut introduction, peanut-induced FPIES seems to be increasing in Australia and USA. Although most patients are diagnosed with FPIES within the first year of life, most commonly with food triggers to cow’s milk or soy, other phenotypes exist in comparison to this classic presentation. In this case report, we present a patient with late onset of acute FPIES at age 3 to walnut.Case PresentationWe describe a case of FPIES in a 12-year-old boy who had recurrent episodes of repetitive emesis, that began at age 3, each time after consuming walnuts. Mom does not give a history of intentional feeding (or avoidance) of walnut and/ or pecans. She also described possible reactions with pine nuts and macadamia. He was assessed with an oral food challenge to walnut, which triggered an episode of acute FPIES. He developed vomiting with onset 2 h post-ingestion, pallor, lethargy, and required an emergency department visit for anti-emetic medications and oral rehydration therapy. He improved on the therapy and now avoids cashew, pistachio, hazelnuts, walnuts, pecans, pine nuts, and macadamia nuts.ConclusionsThis case report adds to the limited literature that exists on culprit food allergens in FPIES. We present an acute FPIES triggered by the ingestion of walnuts. The diagnosis, common food triggers, and natural history of FPIES are described. There remains a lack of information on the natural history of FPIES, especially towards uncommon food triggers and on FPIES that present beyond infancy.