The relationship between occupational exposure and disease that appears many years after exposure has been studied. By necessity, studies of long-term disease are usually restricted to analysis of mortality patterns, and are limited to the fatal diseases in which the mortality provides a good index of disease incidence. 59,000 steal workers employed at 7 plants of 3 major steel firms in Allegheny County, Pennsylvania in 1953 were studied. Employment records on these men were examined in 1962. Statistics were compiled for status of employment, and mortality rates, classified according to cause, status of employment, and race. These were compared with expected values. It was found that coke workers in Allegheny County experienced unusually high lung-cancer mortality. The same was true in other geographical areas. The role of smoking in these cases of lung cancer was considered. However, after a review of the statistics, it was believed that a causal relationship did exist between exposure to coke-oven effluent and lung cancer.