BACKGROUND: Cefaclor is commonly prescribed for various infectious diseases. It is known as a major causative antibiotic that induces drug hypersensitivity reactions. The aim of this study is to evaluate clinical and immunologic features of cefaclor immediate hypersensitivity. METHODS: We conducted a retrospective study on the patients with histories of immediate hypersensitivity to cefaclor at Ajou University Hospital between January 2002 and February 2011. Serum specific IgE to cefaclor was measured using the ImmunoCAP system. RESULTS: Fifty patients were enrolled in this study. The most common immediate hypersensitivity reaction was anaphylaxis (38/50, 76%), followed by urticaria/angioedema (12/50, 24%). High serum specific IgE to cefaclor was noted in 40 patients (80%). There were no significant differences in clinical parameters whether the patients had serum specific IgE or not, except that serum total IgE level was higher in patients with high serum specific IgE to cefaclor (P=0.042). Serum specific IgE to penicillin was detected in 26.1% of the patients with high serum specific IgE to cefaclor, while serum specific IgE to aminopenicillin was not found in any of the study subjects. CONCLUSION: Immediate hypersensitivity to cefaclor occurred mostly through the IgE-mediated mechanism, and anaphylaxis was the most common manifestation. Cross reactivity with penicillin was noted in 26% of the patients, but no cross reactivity with aminopenicillin was noted.