Background: Topical tea tree oil has become increasingly popular as a general-purpose medicament for various dermatological conditions. Although recent reports of adverse cutaneous reactions have highlighted concerns about the risk of inducing acute allergic contact dermatitis, few published studies have attempted to evaluate objectively the true irritancy and allergenicity potential of this commonly used essential oil. Objective: To perform predictive testing for irritancy and allergenicity of tea tree oil in a large group of human subjects. Methods: Various concentrations of tea tree oil (5, 25, 100%) in different vehicles were applied under occlusive patch testing to the skin of healthy, human volunteers (n = 311) using a protocol based on the Draize human sensitisation test. Patch test sites were assessed every 48 h and scored according to the severity of any resulting cutaneous reaction using a clinical grading scale from 0 to 4 over a 21-day induction period. After a 14-day rest period, subjects underwent challenge testing with the same tea tree oil samples to determine if sensitisation to tea tree oil had occurred. Results: The mean irritancy score for each test sample was low, ranging from 0 for 5% tea tree oil to 0.2505 for neat 100% tea tree oil. However, 3 subjects developed grade 3 skin reactions during the induction period suggestive of an allergic reaction. Conclusions: Topical application of tea tree oil is associated with negligible skin irritancy. In the group of subjects studied, the risk of developing an allergic dermatitis from topical tea tree oil usage was found to be <1%.