<b><i>Background:</i></b> Pyrazolones are the most common causes of selective nonsteroidal anti-inflammatory drug (NSAID) hypersensitivity. We studied a large group of patients with immediate and delayed selective responses to metamizole. <b><i>Methods:</i></b> Patients with suspicion of hypersensitivity to metamizole were evaluated. We verified acetylsalicylic acid tolerance and classified patients as immediate or delayed responders if they showed symptoms less or more than 24 h after metamizole administration. Skin tests were performed and if negative, a basophil activation test (BAT) was performed on immediate responders. If it was negative, we performed a drug provocation test (DPT) with metamizole. <b><i>Results:</i></b> A total of 137 patients were included: 132 reacted within 24 h (single NSAID-induced urticaria/angioedema/anaphylaxis; SNIUAA) and 5 after 24 h (single NSAID-induced delayed hypersensitivity reaction; SNIDHR). Most SNIUAA patients developed anaphylaxis (60.60%); for SNIDHR, maculopapular exanthema was the most frequent entity (60%). Skin testing was positive in 62.04% of all cases and BAT in 28% of the SNIUAA patients with negative skin tests. In 5.1% of the cases, DPT with metamizole was needed to establish the diagnosis. In 22.62% of the cases, diagnosis was established by consistent and unequivocal history of repeated allergic episodes in spite of a negative skin test and BAT. <b><i>Conclusions:</i></b> SNIUAA to metamizole is the most frequent type of selective NSAID hypersensitivity, with anaphylaxis being the most common clinical entity. It may occur within 1 h after drug intake. SNIDHR occurs in a very low percentage of cases. The low sensitivity of diagnostic tests may be due to incomplete characterization of the chemical structures of metamizole and its metabolites.