Highlights of the literature on the effects on mortality, reproduction, and morbidity of 2,4,5-trichlorophenoxyacetic-acid (93765) (2,4,5-T), 2,3,7,8-tetrachlorodibenzo-p-dioxin (1746016) (2,3,7,8-TCDD) and other isomers of TCDD and the dioxin nucleus were reviewed. The production of 2,3,7,8-TCDD as an undesirable contaminant of production of 2,4,5-T, and its presence in 2,4,5-T combinations such as Herbicide Orange were discussed. Animal studies have established the extreme toxicity of 2,3,4,8-TCDD, including hepatotoxicity, lymphoid hyperplasia, general debilitation and wasting, skin lesions, carcinogenesis, embryotoxicity, and teratogenicity. Mortality studies in the United States on several small groups of workers exposed to TCDD have not shown a definite excess of cause specific mortality, but the statistical power of the studies and the period of time between exposure and study have been inadequate. Epidemiologic studies conducted in other countries have indicated a significant association between exposure to TCDD and excess mortality and also a possible tumor inducing effect of phenoxy acids. Administration of purified dioxin to pregnant female animals has consistently produced reproductive defects in the offspring, but epidemiologic studies of effects on human reproduction have been inconclusive. Numerous health effects have been associated with TCDD exposure, but with varying certainty. The most definite effects have been chloracne, a persistent skin eruption, and porphyria cutanea tarda, a disorder of hemoglobin metabolism. The authors conclude that a research effort to elucidate effects on humans of dioxin exposure is of critical importance, that controls are needed to prevent industrial accidents resulting in dioxin contamination, and that an informed debate is needed about whether the use of substances with a potential for dioxin contamination should continue and, if so, at what exposure level.