On July 10, 1976, a reactor exploded in Seveso, north of Milan, Italy, sending a cloud of 2,3,7,8-TETRACHLORODIBENZO-P-DIOXIN (TCDD) over al arge area north of Milan. Plants and animaks in the vicinity of the plant were affected, as were humans exposed tro the cloud. Levels of exposures were classified according to three geographic ndar the plant, and attempts were made to assess the risk to the residents of these areas, some of whom were evacuated. Techniques included assessment of soil samples from the various areas for TCDD contamination and, later, for TCDD persistence; a questionnaire survey of residents to determine their exposure to the cloud and to contaminated animals; pathologic srtudies on animals that spontaneously died in the area following the accident; autopsy reports of syndromes exhibited by those animals; and an examination of the general distribution of mortality among various species of animals. These data, along with the incidence of chloracne in the exposed population, were correlated with the territorial distribution of the TCDD derived from the soil samples