Background. Cosmetic products contribute considerably to the incidence of contact dermatitis. In response to a resolution of the Council of Europe, the National Institute for Public Health and the Environment (RIVM) in The Netherlands set up a pilot project to report undesirable effects attributed to cosmetic products. Objectives. To provide an overview of undesirable effects attributed to cosmetic products and to identify the ingredients involved. The information could contribute to the assessment of whether current EU legislation on cosmetics provides adequate protection. Patients/methods. General practitioners, dermatologists and consumers in The Netherlands completed questionnaires on reported undesirable effects of cosmetics. Dermatologists also carried out patch tests and, where necessary, tests with specific batch ingredients of the associated cosmetic product. A website and a public awareness campaign were launched to encourage consumers to report undesirable effects. Results. Between July 2009 and May 2011, the RIVM received more than 1600 reports. Severe undesirable effects were claimed in 14% of the cases. The most frequently reported cosmetic products were make-up and moisturisers, and the most frequently identified allergens were isothiazolinones and fragrance ingredients. Three patients tested positive for co-polymers/cross-polymers. Conclusions. Further investigations are recommended on the prevalence of isothiazolinone-induced allergic contact dermatitis and the allergenic potential of co-polymers/cross-polymers.