The risk of developing various histologic types of bronchogenic carcinoma was determined in a ten-year prospective study of 6,136 older men in relation to smoking habits at the beginning of observation. No lung cancer developed in 830 nonsmokers. Among the 2,580 men who regularly smoked cigarettes only and who were current smokers on entry, the risk of lung cancer increased with increasing daily cigarette consumption. The cancers were typed according to a modification of the World Health Organization classification. Well-differentiated squamous-cell carcinoma, small-cell carcinoma, and adenocarcinoma showed a dose-response relationship to cigarette smoking, but poorly differentiated squamous-cell carcinoma did not.