The case histories of 474 patients with lung cancer were screened. Chronic bronchitis was assumed to precede lung cancer if chronic cough and sputum had been present for more than five years. Daily consumption of cigarettes, cigars (= 5 cigarettes) or pipes (= 1 cigarette) and the number of pack-years were noted. FEV1 in percent of the slowly inspired vital capacity (FEV1 %VC) was regarded as a sensitive index for the degree of bronchial obstruction. Group 1 included 221 lung cancer patients with a clear history of preexisting chronic bronchitis. Group 2 included 175 cases without such history. Heavy smokers and severely obstructed patients were found more often in group 1 than in group 2 (p less than 0.0005). A lack of correlation was evident between smoking habits and FEV1 %VC when group 1 was fused with group 2: r= -0.071, p greater than 0.20. Indeed, light, heavy, and very heavy smokers were evenly distributed in group 1 among severely and slightly obstructed patients. However, in group 2 heavy and very heavy smokers were found more frequently in patients with slight bronchial obstruction than in patients with severe bronchial obstruction (p less than 0.01). It appears that an important subgroup of heavy and very heavy smokers with lung cancer are protected from chronic bronchitis as well as from bronchial obstruction. The reasons for this protection are not clear.