Study summary: Three cases of gynecomastia (enlarged breasts) in prepubertal boys who were otherwise healthy were found to be caused by the topical application of products containing lavender and tea tree oils. The boys were aged 4 years 5 months, 10 years 1 month, and 7 years 10 months. Each boy had normal serum concentrations of endogenous steroids. The products linked to the enlarged breasts were a healing balm containing lavender oil, a hair styling gel and a shampoo both listing Lavandula angustifolia (lavender) oil and Melaleuca alternifolia (tea tree) oil as ingredients, lavender-scented soap and lavender-scented commercial skin lotions. After discontinuing the use of these products, the gynecomastia resolved within several months in each of the patients. Testing using dose-response experiments in human breast cancer cell lines led the authors to conclude that the two oils have estrogenic and antiandrogenic properties.