In 1976, an explosion at the Industrie Chimiche-Meda-Societa Azionaria (ICMESA) plant in Meda, near the Italian town of Seveso, released a cloud of aerosol containing 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), which contaminated the surrounding area. Clean-up of the highly polluted area of Seveso started on May 1980: in order to avoid exposure to significant levels of TCDD, safety measures are being used, but a potential exposure cannot be ruled out. A prospective study was set up at the beginning of the clean-up procedures, and laboratory tests are being used to evaluate trends among the decontamination workers. A reference group who met eligibility criteria at the preemployment examination was selected from employees of the same firm employing the decontamination was selected from employees of the same firm employing the decontamination workers. Medical examinations and laboratory tests have been performed on both the exposed and nonexposed groups at the same facilities. Analysis of the first follow-up period did not show any remarkable changes in a battery of laboratory tests of the exposed and nonexposed groups.