Kathon CG is a preservative, widely used in skin care products and cosmetics. Prior to introduction to the market, extensive studies on the sensitizing potential of this product were performed. The conclusion drawn from these studies was, that the risk of sensitization was extremely small, when used in concentrations below 15 ppm. Now, ten years later, the frequency of allergy to Kathon CG is 1-2% in the Danish population, which is in the same range as the frequencies for the other preservatives. It is concluded, that the preliminary studies were insufficient and monitoring of the prevalence rates of sensitization is important, when new potential sensitizers are introduced. A total ban on the isothiazolinone derivates from the cosmetics, will only transfer the problem to the other preservatives already known as sensitizers. It might be technically acceptable to lower the concentrations further. Since the cosmetic products are not declared, a correct diagnose may be delayed or missed in patients with eczema. Ingredient labelling should be required by the authorities.