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Reeko Osada,1 Chiyako Oshikata,1– 3 Yuichi Kurihara,4 Kosuke Terada,1 Yuka Kodama,1 Yuga Yamashita,1 Ryo Nakadegawa,1 Hinako Masumitsu,1 Yuto Motobayashi,1 Hirokazu Takayasu,1 Nami Masumoto,1,3 Saki Manabe,1 Yingyao Zhu,4 Ryo Tanaka,4 Takeshi Kaneko,3 Aya Sasaki,5 Naomi Tsurikisawa1– 3 1Department of Respirology, National Hospital Organization Yokohama Medical Center, Yokohama, Japan; 2Department of Allergy and Respirology, Hiratsuka City Hospital, Hiratsuka, Japan; 3Department of Pulmonology, Yokohama City University Graduate School of Medicine, Yokohama, Japan; 4Department of Dermatology, Hiratsuka City Hospital, Hiratsuka, Japan; 5Department of Clinical Laboratory, Tokyo Dental College Ichikawa General Hospital, Ichikawa, JapanCorrespondence: Naomi Tsurikisawa, Department of Respirology, National Hospital Organization Yokohama Medical Center, 3-60-2 Harajuku, Totsuka-ku, Yokohama, 245-8575, Japan, Tel +81-45-851-2621, Fax +81-45-851-3902, Email User831328@aol.comAbstract: Monosodium glutamate (MSG), a salt form of a non-essential amino acid, is widely used as a food additive, particularly in Asian cuisines, due to its unique flavor-enhancing qualities. Type I allergic reactions to MSG have not previously been reported. Our patient, a 21-year-old woman, was 14 years old when she first noticed swelling of her tongue (but no oral itching, diarrhea, or abdominal pain) after eating various snack foods. Current skin prick testing elicited a weak positive reaction to MSG. We then performed an oral challenge test during which our patient ingested potato snacks. Subsequent histology showed telangiectasia of the buccal mucosa, interstitial edema in the subepithelial submucosa, and mast cell infiltration. Oral mucosal challenge tests using sodium glutamate confirmed oral swelling in this patient. This report is the first to confirm a case of type 1 allergy to MSG by combining pathology findings with the results of challenge testing.Keywords: food additive, mast cell, monosodium glutamate, food allergy