Humans are exposed to mixtures of polyhalogenated aromatic hydrocarbons, and the potential health effects of these exposures are uncertain. A subset of this class of compounds produce similar spectra of toxicity in experimental animals as does 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and these chemicals have been classified as "dioxins." In this study, we compared the body burdens of dioxins that produce effects in experimental animals to body burdens associated with these effects in humans. Human body burdens were estimated from lipid-adjusted serum concentrations of dioxins, assuming dioxins are equally distributed in body fat and an adult has 22% body fat. The toxic equivalency factor (TEF) method was used to calculate body burdens of dioxins in humans. These calculations included dibenzo-p-dioxins, dibenzofurans, and polychlorinated biphenyls. In the general population, average background concentrations were estimated at 58 ng TCDD equivalents (TEQ)/kg serum lipid, corresponding to a body burden of 13 ng TEQ/kg body weight. Populations with known exposure to dioxins have body burdens of 96-7,000 ng TEQ/kg body weight. For effects that have been clearly associated with dioxins, such as chloracne and induction of CYP1A1, humans and animals respond at similar body burdens. Induction of cancer in animals occurs at body burdens of 944-137,000 ng TCDD/kg body weight, while noncancer effects in animals occur at body burdens of 10-12,500 ng/kg. Available human data suggest that some individuals may respond to dioxin exposures with cancer and noncancer effects at body burdens within one to two orders of magnitude of those in the general population.