Kathon CG, a cosmetics preservative containing, as active ingredients, 5-chloro-2-methyl-4-isothiazolin-3-one and 2-methyl-4-isothiazolin-3-one, appears to be a frequent cause of contact dermatitis in Europe. In the United States, where Kathon CG was introduced some 5 years later, the use of this preservative system for cosmetics and toiletries is rapidly increasing. Undoubtedly cases of contact sensitization will soon emerge in this country. Most cases of contact allergy are caused by the use of moisturizing creams on (slightly) damaged skin. Sensitization by the use of cosmetic products on previously healthy skin, especially the face, does occur but appears to be less frequent. Rinse-off products do not seem to have a substantial potential for the induction and elicitation of contact allergic reactions to Kathon CG because of dilution of the product and the allergen with water as well as a short contact time with the skin. This review provides a synopsis of current knowledge on the preservative system Kathon CG, with emphasis on the risk of sensitization and diagnostic procedures.