Background: Food recalls can play a role in preventing or reducing the number of allergic reactions that may occur after a product containing an undeclared allergen has been introduced into commerce. Objective: We sought to summarize the US Food and Drug Administration's records of recalls classified for fiscal year 1999 involving foods containing undeclared allergens. Methods: Food and Drug Administration food recall records were reviewed for fiscal year 1999 to identify recalls that occurred because of the undeclared presence of one or more of the following allergens: milk, eggs, fish, wheat, crustacean shellfish, tree nuts, peanuts, and soy. Each record was reviewed to determine the recalled product, the undeclared allergen present, the reason for recall, and reported adverse events. Results: Of 659 total food products classified for recall during fiscal year 1999, 236 (36%) products were recalled because they contained one or more undeclared allergens. Consumers were the party most often responsible for identifying that an undeclared allergen was present in a product (56% of recalled products). A total of 34 consumers reported allergic reactions after consumption of the recalled products. Three principal factors contributed to the presence of undeclared allergens in the recalled products: ingredient-statement omissions and errors (51% of all recalled products); manufacturing equipment cross-contact (40%); and errors by ingredient suppliers or manufacturing firm employees (5%). Conclusion: The presence of undeclared allergens in food products represents one of the more common reasons for food-product recall in the United States. A number of well-recognized allergens may be introduced into foods as a result of several different factors. (J Allergy Clin Immunol 2002;109:1022-6.)