The evidence on human carcinogenicity of the common herbicide 2,4- dichlorophenoxyacetic-acid (94757) (2,4-D) was assessed. Topics included: 2,4-D uses and exposure; toxicology (metabolism, acute toxicity, mutagenicity, carcinogen bioassays); epidemiology (case/control studies and cohort studies); and weight of evidence evaluation. Data from occupational exposure studies indicated that 2,4-D was absorbed at varying rates via inhalation, ingestion, and dermal absorption. The latter accounted for about 90% of the total absorbed dose in workers who sprayed herbicides, whereas inhalation was more important in chemical manufacturing workers. The main symptoms of acute toxicity included damage to muscle tissue, the gastrointestinal tract, and the central nervous system. Mutagenicity tests were generally negative. No statistical increases in the incidence of tumors were reported. Studies hinged around the development of soft tissue sarcomas (STS), Hodgkin's disease (HD), and nonHodgkin's lymphoma (NHL) on exposure to the chemical. The case/control studies did not show an association between 2,4-D exposure and STS and HD. However, the findings for NHL suggested an increased risk of NHL with increasing years of exposure. Cohort studies provided little evidence to suggest increased risk for the more common cancers, but the findings regarding the less common cancers was problematic. The authors indicate the potential cancer risk for people and recommend the use of protective equipment while mixing and applying herbicides.