A patient with recurrent idiopathic urticaria reported exacerbations after treatment with cetirizine. Prick test to cetirizine was negative. Double-blind challenge tests with mizolastine, loratadine, fexofenadine, dexchlorpheniramine, ebastine, ketotifen, and placebo were negative, whereas hydroxyzine and its active metabolite, cetirizine, reproduced the urticaria. Identification of uncommon adverse reactions to H1 antihistamines is important, particularly because they may mimic the underlying disease.