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Temporary henna tattoos are becoming increasingly popular among Western tourists during summer holidays, especially children, teenagers and young adults. Natural henna takes several hours to be absorbed into the skin, imparts a brownish-orange color, and causes very few allergic reactions. Para-phenylenediamine (PPD), a powerful allergen, is added to henna tattoo mixtures (black henna tattoo) to decrease application time and intensify the color. It is responsible for most of the complications reported after henna tattoos: localized or generalized contact dermatitis, hypertrophic or keloid scars, and temporary or permanent hyper- or hypopigmentation. More rarely, type I hypersensitivity reactions (urticaria, angioedema, or anaphylaxis) with potentially lethal outcomes have been reported. PPD allergy can induce cross-reactivity with other substances, such as hair dyes and textile azo dyes. Patch testing for PPD must be performed at a very diluted concentration (0.01% in vaseline) to avoid unnecessarily strong reactions and sensitization to PPD. In the absence of any legal control of henna tattooing practices, prevention requires the annual provision of information to Western consumers, especially young people and their parents.