Food allergy can be diagnosed from an immediate-onset reaction after oral food challenge. We administered 370 challenges in 242 children; five (3%) of the 193 children tolerating foods on challenges developed immediate symptoms when the same preparation of foods was ingested openly at home the next day. We confirmed the food allergy by subsequent double-blind challenges. All children had positive skin-prick test reactions and all, but one, specific IgE antibodies to the offending foods. Our findings suggest that open feedings under observation, the day after negative challenges, are useful to eliminate false-negative challenge results.