A case of severe systemic reactions (intense itching, urticaria, confusion, blurred vision, transient loss of consciousness, sweating, tachycardia) after ingestion of raw or lightly-cooked onion is described. The patient, a 44-year-old woman, had no troubles with well-cooked onions. Differently from the cases of sensitivity to onion described in literature, this patient was monosensitized, being skin tests negative to pollens, inhalants and other foods. The patient had 3.7 kU/L of onion-specific segum IgE, as determined by REAST. The density of onion-specific IgE (calculated as percent ratio to total IgE) was 30.8%. The reactivity of patient's serum IgE towards thermolabile and thermostable components has been tested with unheated and heated (30' at 100 degrees C) onion extracts bound to polystyrene beads and tested in the RAST system. Unheated extract resulted positive in class 2, heated extract negative, demonstrating that this patients, differently from similar clinical cases described in literature, had IgE antibodies recognizing just thermolabile onion fraction. This is the first case described in literature of a monosensitization to the thermolabile component of onion, negative also to related foods (Liliacee) and characterized by severe systemic reactions. The importance of specific-IgE density (%) rather their absolute amount (kU/L) as parameter predictive for the clinical severity of allergic reactions is discussed.