For men participating in the Multiple Risk Factor Intervention Trial, the authors evaluated the relation between the baseline forced expiratory volume in one second and lung cancer mortality among smokers between the third and tenth years of follow-up (1973-1974 to 1984). This measure of ventilatory function was a powerful predictor of lung cancer deaths, with rates that increased from 3.02 per 1,000 person-years in the lowest quintile of forced expiratory volume to 0.43 in the highest quintile. This relation was not weakened by adjustments for smoking dose, including number of cigarettes smoked per day, tar and nicotine content, duration of smoking, or age at onset of smoking. Nor was forced expiratory volume related to time between its determination and lung cancer death. If these observations can be verified in other studies, the forced expiratory volume in one second may identify smokers at very high risk of lung cancer and, hence, in need of more aggressive preventive approaches.