Background: Human subjects can be parasitized by Anisakis simplex by eating raw or undercooked fish. Gastric anisakiasis is probably the most frequent clinical entity presenting with severe epigastric pain, vomiting, and diarrhea. In gastroallergic anisakiasis hypersensitivity symptoms predominate. Objective: We sought to describe clinical features, laboratory data, and gastroscopic findings in gastroallergic anisakiasis. Methods: We selected 40 patients presenting to the emergency department with an acute allergic reaction, and if we suspected acute parasitism by A simplex , a fiberoptic gastroscopy was performed. In 20 patients we could detect one or more nematodes; these patients are referred to as group A. Those in whom no worm could be found are referred to as group B (n = 20). A detailed history, clinical features, gastroscopic findings, laboratory data, and skin prick test responses were compared. Results: Long-time intervals of up to 26 hours between fish intake and onset of hypersensitivity symptoms were found (group A, 5.4 ± 6.3 hours; group B, 5.3 ± 2.6 hours). Patients in groups A and B did not differ with respect to allergic symptoms (urticaria, angioedema, erythema, bronchospasm, and anaphylaxis) or the mainly light abdominal symptoms (upper abdominal pain, nausea, vomiting, and diarrhea). No significant differences were found with respect to age, time interval between fish intake and onset of symptoms, white cell and eosinophil counts, specific IgE levels against A simplex , or total IgE levels. Conclusions: The peculiar sometimes long-time interval between fish intake and onset of allergic symptoms render the diagnosis difficult. An early gastroscopy can confirm the diagnosis and prevent complications. We suggest that gastroallergic anisakiasis be considered a distinct clinical entity in which the predominant symptoms are hypersensitivity symptoms and in which the correct diagnosis is not only important in the management of the acute reaction but also in the prevention of further allergic episodes. (J Allergy Clin Immunol 2000;105:178-81.)