BACKGROUND The administration of aspirin and other nonsteroidal anti-inflammatory drugs in patients sensitive to nimesulide might be hazardous. OBJECTIVE To assess the tolerance to both acetaminophen (paracetamol) and aspirin in patients with a history of urticaria induced by nimesulide. METHODS Nine patients with a history of nimesulide intolerance were submitted to single-blind, placebo-controlled peroral challenges with increasing doses of acetaminophen and aspirin. RESULTS Acetaminophen was tolerated by all patients, whereas two experienced immediate systemic urticaria after the administration of 125 mg of aspirin. CONCLUSIONS Acetaminophen and aspirin are well tolerated by most nimesulide-sensitive patients. Since a minority of patients show aspirin sensitivity, tolerance of this agent should always be ascertained by properly performed peroral challenges.