Contact dermatitis among hairdressers is common. In The Netherlands, registered sick leave for hand dermatitis among hairdressers rose from 21,050 days in 1986 to 54,293 in 1991. In a survey among 45 hairdressers in 5 different salons, 12 had a history of hand dermatitis and 16 showed moderate to severe hand dermatitis. After extensive investigations, 13 were classified as having allergic contact dermatitis and 3 cumulative irritant contact dermatitis. In the past 4 years, 103 hairdressers were extensively patch tested and glyceryl thioglycolate (GTG), ammonium persulfate and nickel sulfate were responsible for the majority of positive reactions. Hair dyes and preservatives were responsible for a moderate % of the positive reactions. Positive reactions were also found to cocamidopropyl betaine and sodium coco hydrolyzed animal protein. These 2 allergens show a rather capricious patch test reaction pattern and irritant reactions may easily be confused with allergic. The relevance of positive patch test reactions to these chemicals should always be questioned. Atopy was not a frequent cause of hand dermatitis in this study. Chemicals with a thiol group can be demonstrated with a chemical spot test. With this test, contamination of the hairdressing salon with thioglycolates was demonstrated. It is emphasized that contamination of hairdressing salons with GTG is probably a significant factor in explaining the severe flare-ups in GTG-sensitized hairdressers who no longer use GTG permanent-waving solutions.