Cephalosporines are the most important β-lactams inducing IgE-mediated reactions such as urticaria, angioedema and anaphylaxis. There have been a few reports that describes assays of serum specific IgE for cephalosporins. We experienced a case of cefaclor-induced anaphylaxis and detected serum specific IgE to cefalor-human serum albumin (HSA) conjugate. A 40-year old man was hospitalized due to sudden dyspnea. generalized urticaria, facial edema 30 minutes after taking cefaclor (250mg, po) for an upper respiratory infection. His systolic blood pressure dropped to 50 mmHg at admission. He had been treated with cefaclor for chronic osteomyelitis of the right heel for 1 year without adverse reactions. He had no personal or family history of allergic disease nor penicillin hypersensitivity. Skin prick test with cefaclor showed a positive response and serum specific IgE to cefaclor-HSA conjugate was detected by ELISA inhibition test showed significant inhibitions with additions of cefaclor-HSA conjugate and cefaclor in a dose-dependent manner. In conclusion, we confirmed that cefaclor-induced anaphylaxis in this patient was an IgE-mediated reation to cefaclor-HSA conjugate.