Dear Editor, Exposure to hair-dye is the most frequent route of sensitization to para-phenylenediamine (PPD), a common contact allergen (1). Patients allergic to PPD often worry about the use of hairdressings and usually ask about safe alternative agents and the use of other hairdressing materials such as permanent wave agents, shampoos, and bleaching agents. However, there have been no studies on this issue. The aim of the present study was to investigate the positivity of hair-related allergens and identify safe alternatives. We conducted a prospective patch test study using the hairdressing series (Chemotechnique Diagnostics®, Vellinge, Sweden) in patients allergic to PPD and control subjects. The control group was defined as subjects who had no history of allergic contact dermatitis (ACD) to hair dye and negative reaction to PPD. Volunteers in the experimental group were recruited from patients who were previously diagnosed with PPD-induced ACD using a TRUE test® and clinical relevance, but hairdressers and barbers at high risk of being exposed to hairdressing materials were excluded. The study protocol was approved by the local ethical committee, and informed consent was obtained from all volunteers (PNUH IRB No. F2010004). We tested 24 patients allergic to PPD and 21 control group subjects; 20 of the former and 19 of the latter completed the study. A summary of the study results is shown in Table 1. Eleven patients from the experimental group (55.0%) were positive for more than one allergen other than PPD, while only 2 from the control group (11.1%) were positive (P<0.05). Para-toluenediamine sulfate (15.0%), 3-aminophenol (10.0%), and nickel-sulfate hexahydrate (10.0%) were common allergens, which is consistent with the findings of other studies that investigated hairdressing allergens (2). The rates of positive reaction for each allergen showed a higher frequency than in the control group. Previous studies reported that people allergic to PPD were also positive for other hair-related allergens (2-4). We thus suggest that people who are allergic to PPD should be cautious in using hairdressing materials and recommend performing patch tests to identify safe agents. Based on these results, although the positive rates were not statistically significant, patients in our case should also be cautiously tested with other hair-dye components. We also tentatively hypothesize that getting a permanent wave is safer than a hair-dye in patients allergic to PPD, because all study patients showed negative reaction to permanent wave components. Although our study was too small in number to generalize the results, it is a unique study on PPD-induced ACD in patients from the general population examining the positivity of each allergen in the hairdressing series. Furthermore, while most studies on the topic were of retrospective design, our study was a prospectively designed study. The patients allergic to PPD were advised to perform hairdressing series patch test and to use other hairdressing materials with care, such as bleaching agents, shampoos, and especially hair-dye components other than PPD, due to cross-reaction. Moreover, we expect this study could be an important preliminary study offering clinical data on cross-reaction with PPD and other hair-dye components.