A review of the data and a summary of findings regarding the potential human health hazards of 1,3-butadiene (106990) were presented. Background data included physical properties, chemical properties, production, use, and potential for occupational exposure. Data compiled from the NIOSH National Occupational Hazard Survey indicated that about 65,000 workers were potentially exposed to 1,3-butadiene; of these, almost 45,000 were in the chemical and allied products industry. The OSHA 8 hour time weighted average concentration was 1,000 parts per million (ppm). The American Conference of Governmental Industrial Hygienists had indicated that it intended to change the threshold limit value for 1,3-butadiene based on carcinogenic potential. Experimental studies had indicated that 1,3-butadiene induced a carcinogenic response in rats and mice at multiple sites. Rats exposed at 1,000 to 8,000ppm developed mammary, uterine, Leydig cell, thyroid follicular cell, pancreatic exocrine and Zymbal gland tumors, while mice exposed to 625 or 1,250ppm showed an increased incidence of malignant lymphomas, hemangiosarcomas, and other tumors. Testicular and ovarian atrophy was also seen in mice. Exposure of pregnant rats to 1,3-butadiene at 8,000ppm produced major skeletal defects in offspring; exposure to 200 to 8,000ppm produced fetal toxicity. Results of epidemiological studies indicated that there was a nonstatistically significant excess mortality for lymphatic and hematopoietic neoplasms, and leukemia. NIOSH recommends that 1,3-butadiene should be considered a potential carcinogen, and teratogen, and as a possible reproductive hazard. Worker exposures should be reduced by appropriate engineering controls and work practices.