Oxidative hair dyes have repeatedly come to the attention of the dermatologic community owing to concerns about contact dermatitis. A review of the scientific literature provides insight into the prevalence of p-phenylenediamine (PPD)-sensitized individuals and on the prevalence of hair dye dermatitis in various types of patient and nonpatient populations mainly from Europe and from the United States and Asia. Most of the results are obtained through patch testing with PPD. PPD is one of the main oxidation colorants; however, patch-test prevalence of PPD is not equivalent to prevalence of hair dye allergy. An analysis shows no clear increase in the frequency of positive patch-test reactions to PPD in eczema patients and in the general population. All the parameters through which the frequency of hair dye dermatitis resulting from exposure to PPD is evaluated have been stable in Europe, with a few exceptions that are discussed in the review. There is a statistically significant decrease (p < .0001) in the prevalence of positive patch-test reactions to PPD in North America (1970 to 2002). Data from studies in Asia are difficult to interpret. Pooled prevalence rates of positive patch-test reactions to PPD were calculated for the three continents.