BACKGROUND The ingestion of Anisakidae ssp larvae parasitized fish can cause anisakiasis. Allergic reactions after ingestion of safely cooked but parasitized fish have been reported. METHODS Twenty-three patients who suffered allergic reactions after seafood ingestion, with negative skin tests were studied. Anisakis simplex sensitization was assessed by skin prick test and/or specific serum Immunoglobulin E (IgE). Total serum IgE and specific IgE against the implicated seafood and Ascaris lumbricoides were also determined. RESULTS Manifestations of Anisakis simplex allergy were urticaria/angioedema (18/23) patients and anaphylaxis (5/23). Gastric symptoms were also observed (3/23). Sea fish and shellfish were implicated. Raw and cooked seafood ingestion caused reactions. Total serum IgE ranged from 13 to 7200 KU/L. Specific IgE to Anisakis simplex was positive (> 0.35 KU/L) in all patients, and skin tests were positive in 20. Serum-specific IgE and skin tests to the involved seafoods were negative in every patient. Serum-specific IgE to Ascaris lumbricoides was negative in 13 patients. No association between total IgE and the eosinophil count (r < 0.1) was observed, but there was some association between total IgE and specific IgE to Anisakis simplex (r = 0.58). CONCLUSIONS Anisakis simplex sensitization is the cause of allergic reactions after seafood ingestion. It is important to pay attention to this new "food allergy" to diagnose correctly the etiology of adverse food reactions.