During 1965 to 1968, 80 workers who had been engaged in the production of 2, 4, 5-sodium trichlorphenoxyacetate and butylester of trichlorphenoxyacetate acid became ill. The cause of the illness was 2, 4, 7, 8-tetrachlordibenzo-p-dioxin. A 10-yr study has been conducted for 55 exposed individuals. The majority of the patients developed chloracne, and 11 manifested porphyria cutanea tarda. Approximately one-half of the patients suffered from metabolic disturbances, i.e., pathologically elevated lipids with abnormalities in the lipoprotein spectrum, and two-fifths of the patients had pathological changes in the glucose tolerance test. One-third of the patients had biochemical deviations indicative of a mild liver lesion. Histological examination revealed light steatosis, or periportal fibrosis, or activation of Kupffer cells. Fluorescence of the liver tissues was present in ultraviolet light. In 17 persons symptoms of nervous system focal damage existed, with predominance of peripheral neuron lesion of the lower extremities (verified by EMG examination). The majority of patients suffered from various psychological disorders. As of this date, two patients have died of bronchogenic lung carcinoma; one of liver cirrhosis; one of a rapidly developed, extremely unusual type of atherosclerosis precipue cerebri; and two patients have died in traffic accidents. The conditions of most other patients have improved.