We present a 48-year-old patient who complained for 1 year about urticarial reactions which appeared always when he ingested alcoholic beverages. Skin prick tests with ethanol were negative but positive with 10% acetic acid in the patient. Normal controls tested negative with acetic acid. Skin prick tests to common immediate-type allergens were negative. The patient underwent a double-blind, placebo-controlled challenge test. A few minutes after challenge with ethanol but not with placebo, the patient developed erythema and wheals on the chest and the upper arms. The tryptase serum level rose from undetectable (0.1 U/ml) before challenge to 3.8 U/ml after skin lesions had appeared. This case demonstrates that increased tryptase serum levels can help in the diagnosis of ethanol-induced urticaria.