Anisakis simplex is a common parasite in fish and cephalopods and is not only capable of causing anisakiasis in humans through visceral invasion of the third-stage larvae but can also cause anaphylactic reactions, as has recently been demonstrated. We present the clinical case of a 56-year-old man who initially presented anaphylactic reactions related to eating fish. Shortly afterwards, he began to experience self-limiting recurrences of very intense epigastric pain, nausea and vomiting. Skin tests for immediate hypersensitivity (prick tests) with a commercial extract as well as the determination of specific IgE in the patient's serum were clearly positive for A. simplex. The hemogram did not show eosinophilia. Copro-cultures and parasites in the patient's feces were repeatedly negative. Gastroscopy was normal. The intestinal tract showed contrast flocculation and dilation of ansas in the distal duodenum and proximal jejunum. Biopsy samples of gastric and distal duodenum mucous showed an active process of chronic inflammation with a predominance of eosinophils in the lamina propria. After subjecting the patient to a fish and cephalopod-free diet and treating him with thiabendazole 350 mg every 12 hours for 6 days, he showed no sign of symptoms while awaiting new tests. Even though the diagnosis of IgE-mediated allergy caused by A. simplex offers no room for doubt, we are unable to present a firm diagnosis of anisakiasis as no larva has been seen. Nevertheless, the clinical pattern, the image of the intestinal tract, the eosinophilic infiltrate in the biopsies and the good response to thiabendazole all lead to the suspected existence of anisakiasis in this patient coexisting with IgE-mediated allergy to this parasite.