Background: Isopropylantipyrine is a pyrazolone derivative, a kind of NSAIDs, however, the immediate hypersensitivity to isopropylantipyrine has been rarely reported. Objective: We evaluated the clinical characteristics of immediate hypersensitivity to isopropylantipyrine. Method: Eleven patients with immediate hypersensitivity reactions to isopropylantipyrine were enrolled. They were classified into two groups: group I consisted of 7 patients with selective hypersensitivity; group II included 4 patients combined with other NSAIDs hypersensitivity. Skin prick and intradermal tests, and oral provocation test with isopropylantipyrine were performed. To confirm selective hypersensitivity, oral provocation test to aspirin was done. Result: The mean age was 35.3 yrs with female predominance (81.8%). The onset age of isopropylantipyrine hypersensitivity was 27.7 yrs. Mean response time after drug exposure was 0.9±0.8 hr and atopic diathesis was found in 5 patients (45.5%). The most common presenting symptoms were skin (90.9%) followed by respiratory (45.5%), ocular (18.2%). cardiovascular (18.2%), gastrointestinal (18.2%) reactions and anaphylaxis (18.2%), with no difference between the two groups. All subjects in both groups showed negative results to skin prick and intradermal tests to isopropylantipyrine. The onset age tended to be younger (22.9 vs. 36.3 yrs), and response time after drug exposure (0.6 vs. 1.5 hrs) tended to be shorter in group I than in group II. Conclusion: We report two groups of isopropyantipyrine hypersensitivity, selective and cross intolerant with NSAIDs groups. Two possible mechanisms were suggested as inhibition of cyclooxygenase activity in cross intolerant group, non-IgE mediate response in selective hypersensitivity group.