Background: Folic acid (the synthetic form of folate B vitamins in foods) is widely used in vitamin supplements. Anaphylaxis from ingestion or injection of folic acid suggests IgE antibody–mediated mechanisms, but this has not been demonstrated previously in vitro. Objective: This study was conducted to better define the mechanism of folic acid hypersensitivity and cross-reactivity among folic acid congeners. Methods: Skin testing was performed with folic acid congeners in a woman who developed anaphylaxis after ingestion of 2 different multivitamin preparations containing folic acid. In vitro immunologic serum studies were conducted using a folate–human serum albumin (HSA) conjugate prepared by a novel application of carbodiimide condensation. Results: The patient had positive immediate-type skin test reactions to folic acid and several folate analogues including leucovorin (folinic acid). Urticaria developed during graded oral test dosing with leucovorin. Using a dot immunoblot assay or an ELISA for IgE antibody to folate-HSA, results of the patient's serum testing were positive, whereas results of sera from normal control subjects were negative, the first in vitro demonstration of IgE to a folic acid–protein conjugate. By ELISA, the positive result of the patient's serum was inhibited significantly by serum coincubation with folate-HSA, but not HSA or folic acid. Conclusions: Immediate hypersensitivity to folic acid and possibly other vitamins can be mediated by IgE antibody to conjugates formed between vitamins and self-proteins or polypeptides. Leucovorin can have clinically important immunologic cross-reactivity with folic acid. A diet rich in natural folates (pteroylpolyglutamates) appears useful as a management strategy for providing adequate nutrition to patients with folic acid hypersensitivity. {J Allergy Clin Immunol 2000;106:386-9.)