Australian tea tree oil has been used as a veterinary antiseptic for many years and, more recently, has been extended into human use. There have been many reports of allergic contact dermatitis and toxicity reactions, but it has never been implicated in immediate systemic hypersensitivity reactions. A 38-year-old man experienced immediate flushing, pruritus, throat constriction, and lightheadedness after topical application of tea tree oil. Our purpose was to determine whether this represented an immunoglobulin E (IgE)--mediated reaction. Skin-prick and intradermal testing was performed, as well as enzyme-linked immunosorbent assays for specific IgG and IgE against tea tree oil. The patient had a positive wheal and flare reaction on intradermal testing with tea tree oil. All five patient controls were negative on skin testing. No specific IgG or IgE was detected. We present the first reported case of an immediate systemic hypersensitivity reaction occurring after topical application of Australian tea tree oil, confirmed by positive wheal and flare reaction on skin testing.