In 1979, a mass poisoning involving some 2,000 persons occurred in central Taiwan from cooking oil contaminated by polychlorinated biphenyls (PCBs) and their heat-degraded byproducts, including polychlorinated dibenzofurans (PCDFs). The responsible health department registered cases for clinical purposes between 1979 and 1983. The exposed persons are referred to as the "yucheng" (oil disease) cohort. PCBs and PCDFs are toxic chemicals widely dispersed in the environment and in human tissue, which persist long after exposure. The consequences of exposure to these agents are not well understood. We traced the cohort through December 31, 1991, and compared overall and cause-specific mortality of 1,837 "yucheng" subjects with age, gender, and calendar time-specific mortality rates for the Taiwan general population. Eighty-three deaths were identified from 23,404 observed person-years. Even though the overall standardized mortality ratio (SMR) was 0.8 (95% confidence interval (CI) 0.7-1.0), there was a substantial elevation in the mortality rate for chronic liver disease and cirrhosis (10 deaths, SMR = 2.7, 95% CI = 1.3-4.9). Mortality from malignant neoplasms and other causes was not significantly different from that of the Taiwan population. PCB/PCDF exposure appears to promote the development of severe liver disease, perhaps in combination with known risk factors such as infection with hepatitis B virus. Further follow-up of this young cohort is necessary to see if the consequences include hepatic cancer.