2,3,7,8-Tetrachlorodibenzodioxin (TCDD) was formed in an uncontrolled decomposition reaction at BASF Aktiengesellschaft on 17 November 1953. This report presents the findings of a 34-year mortality follow-up study of 247 workers who were partly heavily exposed; 78 thereof had died. We divided these employees into three cohorts based on the amount and reliability of the exposure information. The mortality was compared with the national mortality rates in the Federal Republic of Germany (FRG) and is presented in terms of standardized mortality ratios (SMRs) together with 90% confidence intervals (CI) for different periods of time since the first exposure. In general, the overall mortality of these workers was similar to the rates of the national population. The SMR for all malignant neoplasms based on 23 deaths was 117 (90% CI: 80, 166), suggesting no overall increase in cancer among those employees. When workers with chloracne were examined separately, the SMR for all malignant neoplasms was not significantly elevated overall (SMR 139; 87, 211), but it was for the time period 20 or more years after the first exposure (SMR 201; 122, 315). Results for 22 causes of death as well as additional information on the type of exposure and skin findings are presented and discussed in relation to the current literature. In general, our results do not appear to support a strong association between cancer mortality and TCDD, but they do suggest that some hazard may have been produced.