NIOSH has warned that workers exposed to o-toluidine (95534) or aniline (62533) may be at increase risk for developing bladder cancer. Studies on the carcinogenic effects of o-toluidine and aniline in humans and animals were reviewed, and information was presented on the number of workers potentially exposed, and on exposure limits. Epidemiological evidence clearly associated exposure to o-toluidine and aniline with an increased risk of bladder cancer. Risk increased with duration of exposure. Effects of o-toluidine and aniline could not be separated in epidemiological studies. Because o-toluidine induced a greater number and variety of tumors in animals at lower doses than aniline, it was more likely to be responsible for the observed bladder cancers. Recommendations were offered for hazard awareness, training, engineering controls, work practices, the use of personal protective equipment, biological monitoring, and medical screening.