The medical records of 6,506 deaths among railroad workers included 818 deaths from malignant disease of various organs, and 154 of the deaths were ascribed to primary carcinoma of the lung and/or bronchus. The 154 deaths were classified according to the amount of exposure to exhaust fumes from diesel engines involved in the decedent's actual day's work. The group of workers regularly exposed to diesel exhaust, especially engineers and firemen, proved not to have any higher rate of bronchopulmonary carcinoma than did the nonoperating groups, which included clerks and janitors. Paradoxically, the highest rate was in the clerical group, which also contained a higher percentage of urban as distinguished from rural residents. The place of residence was more significant than the type of work. The cases of bronchopulmonary carcinoma here studied all occurred in men even though about 4 % of the employees in the department concerned were women.