Hair cosmetics, in particular hair dyes, bleaches, and permanent wave solutions, contain a number of potential contact allergens. The pattern of exposure to these in hairdressers, clients and home-users may change over time. Data on female patients who consulted centers of the Information Network of Departments of Dermatology (IVDK, http://www.ivdk.org) between 01/2003 and 12/2006 was analyzed. Patients were divided into a) hair-dressers, given they worked in this occupation and suffered from occupational skin disease (n = 480),and b) "clients",who never worked as hairdressers, but in whom hair cosmetics were considered a potential cause of contact dermatitis (n = 780). In hairdressers, the most important allergens were ammonium persulfate (21.7% positive),p-toluenediamine (PTD,19.6% positive),p-phenylenedi-amine (PPD,18.1% positive) and, with decreasing time trend, glyceryl monothioglycolate (2005/2006 still 7.5% positive). Biocides must be regarded as important allergens in hairdressers, namely,(chloro) methylisothiazolinone and methyldibromoglutaronitrile. Clients, who were considerably older on average, had possibly been tested in a more aimed fashion, yielding (strikingly) higher sensitization prevalences to a number of allergens of oxidative hair dyes such as PPD or PTD. The continual analysis of sensitization prevalences in patients patch tested can identify areas needing in-depth research and preventive intervention, such as avoiding the use of certain biocides in shampoo intended for professional use by hairdressers or further reducing the concentrations in oxidative hair dyes.