The chronic health effects of occupational exposure to 2,3,7,8- tetrachlorodibenzo-p-dioxin (1746016) (TCDD) are discussed. Concern about delayed toxic effects that could result from exposure to TCDD is widespread, since a variety of groups have been exposed to it. TCDD is formed during the synthesis of 2,4,5-trichlorophenoxyacetic- acid from trichlorophenol. It is also generated when trichlorophenol is synthesized from tetrachlorobenzene. Acute toxicity studies have shown that TCDD is lethal to some species at concentrations below 1 microgram per kilogram. TCDD has been found to be teratogenic and fetotoxic in female mice and carcinogenic in rats and mice. Preliminary data collected by NIOSH suggest a possible association between occupational exposure to TCDD and an increased number of deaths from soft tissue sarcoma. Many epidemiologic studies have indicated that there is a range of toxic effects associated with TCDD exposure in humans, especially heavily exposed workers. Chloracne, signs of hepatotoxicity such as porphyria and metabolic and lipid abnormalities (including elevated triglyceride concentrations), neurotoxicity, and immunologic changes have been observed. Except for chloracne, the persistence of each of these findings requires confirmation. Other studies have given conflicting data about a possible association between TCDD exposure and heart disease. The authors recommend that additional studies on immunologic, hepatic, neurologic, and reproductive consequences of exposure to TCDD be conducted and that the question regarding human carcinogenicity of TCDD be resolved.