In February, 2008, 17 scientists from seven countries met at the International Agency for Research on Cancer (IARC), Lyon, France, to reassess the carcinogenicity of some aromatic amines and organic dyes. Additionally, occupational exposures of hairdressers and barbers and exposure from personal use of hair colourants were reviewed. These assessments will be published as volume 99 of the IARC Monographs. Some aromatic amines have long been known to cause bladder cancer in human beings, and several others are suspected of being carcinogenic. Aromatic amines also cause bladder cancer whenever they are tested in dogs. The Working Group identified several additional aromatic amines that are carcinogenic to human beings. ortho-Toluidine is used in the production of dyes, pigments, and rubber chemicals, and as a biological stain. It is present in the urine of most people, but its origin remains unknown. In patients treated with the anaesthetic prilocaine, ortho-toluidine was detected in urine as a metabolite. Occupational exposure occurs mainly via dermal contact, and occupational cohort studies have reported greatly increased risks of bladder cancer, which could not be explained by smoking. In two of these studies, and known bladder carcinogens were present only in trace amounts and the highest risk of bladder cancer was noted in the subgroup with the longest duration of exposure. Furthermore, ortho-toluidine-haemoglobin adducts have been identified in exposed workers, as well as in patients treated with prilocaine. ortho-Toluidine also causes tumours at many sites in rodents, including bladder tumours in female rats. ortho-Toluidine was classified as carcinogenic to humans(Group 1).