The major result of this study is that confirmed drug hypersensitivity reactions occurred in less than one quarter of patients with a history suggesting possible drug allergy. A negative drug provocation test result is important to the patient with suspected drug allergy because nonhypersensitive patients do not need to avoid these drugs in the future. The clinical histories of the patients with positive drug provocation test results were more suggestive of drug hypersensitivity reactions than the clinical histories of the patients with negative drug provocation test results. Indeed, 64.7% of the former reactions occurred within the first 8 hours after the last intake of the drug (as compared with 42.7% for the latter) and 15.7% of those reactions were anaphylactic (as compared with 7.3%). However, on a single patient basis, relying on the history to prove drug hypersensitivity (or its absence) is not possible. Similarly, although the proportion of positive test results was higher for aspirin (47.2%), other NSAIDs (27.3%), and quinolones (27.3%) than for paracetamol (16.9%), macrolides (13.7%), and β-lactams (8.4%), this information is not helpful diagnostically in a particular patient. Therefore, a complete drug allergy work-up is required: a detailed clinical history and physical examination, followed by 1 or more skin tests and drug provocation tests (6). Other laboratory-based tests that could possibly replace drug provocation tests are being studied. The drug provocation test result may have been falsely negative in some patients with mild sensitivity or a long delay between drug hypersensitivity reaction and allergy evaluation (spontaneous desensitization). Because of this possibility, we enroll our patients in a follow-up program; no patient has reported an allergic reaction to a drug that we had exonerated on the basis of a negative drug provocation result. If not included in the diagnostic procedure, important cofactors, such as exercise or sunlight, may sometimes explain the negative results (6). Tolerance induction during the provocation, although never documented in the literature, may also explain a false-negative result.