During the period from 1948 to 71, some workers in a chemical plant were exposed to chloromethyl ethers in varying degrees and for varying durations. A historical cohort study of 1,446 male production employees from 1960 through 1975 showed both an increased risk of lung cancer as compared to that in the general population among 465 exposed workers and a dose-response relationship. Significantly increased risk occurred only among men with moderate and heavy chemical exposure. Smoking habit could account for some of the cases of lung cancer that developed in men with light chemical exposure and for cases among unexposed workers. Microscopic sections of most of the lung cancers were reviewed, and the results suggested that small-cell carcinoma was a specific response to inhalation of chloromethyl ethers.