A detailed histological analysis was carried out of 171 cases of lung carcinoma of nonmetastatic origin, selected from the National Coal Workers Autopsy (NCWAS) study. Of these, 160 had been smokers at some time previous to death. Tumors were classified basically according to the World Health Organization method of classification. The relative frequencies of cell types, squamous, small cell undifferentiated, large cell undifferentiated and adenocarcinoma, were used to evaluate the effect of exposure to coal mine dust on histogenesis. The predominant cancer in this group was squamous cell carcinoma, followed by adenocarcinoma and small cell carcinoma in nearly equal proportions. The most frequent of 13 mixed tumors was adenosquamous carcinoma. Adenocarcinoma tended to arise peripherally while squamous cell carcinoma tended to form in the larger airways. Small cell carcinoma arose mainly in the lobar and segmental bronchi. An increasing incidence of squamous cell carcinoma occurred among older miners, with the reverse being true for small cell carcinoma. Frequencies of adenocarcinoma were similar in the different age groups. With increase in number of pack years, smokers showed an increase in the proportions of squamous cell carcinoma, large cell and mixed cell carcinomas. The reverse was true for small cell carcinoma, while adenocarcinoma was not affected by smoking. There was no relationship between years of underground experience, with its associated exposure to coal dust, and the incidence of lung carcinoma. The authors conclude that lung carcinoma in coal miners does not differ significantly from that of smokers in the general population. Coal mine dust exposure has no demonstrable carcinogenic effect.