The draft report reexamines the scientific basis and methods used by the U.S. Environmental Protection Agency (EPA) for estimating the cancer potency of 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD). It focuses on the hazard identification and dose-response assessment for the potential human carcinogenicity of this chemical. The objective of the report is to determine if EPA's 1985 cancer risk assessment for polychlorinated dibenzo-p-dioxins should be modified in light of recent data, alternative risk assessment methods, or alternative interpretations of data on the chemical. The draft report concludes that (1) the 1985 assessment that associates a 0.006 pk/kg/day dose with a plausible upper bound increased cancer risk of one in a million 10 to the -6 power should be reconsidered, and (2) a change to a 0.1 pg/kg/day dose as a plausible upper bound associated with an increased lifetime risk of one in a million is consistent with the available data and theories, and represents a reasonable science policy for the Agency. Appendices A through F to the document contain important background information on a broad range of issues related to 2,3,7,8-TCDD including quantitative implications (A), epidemiology (B and D), reproductive and developmental toxicity (C), immunotoxicity (E), and mechanisms of action (F).