A total of 477 cases have been followed, presenting with chronic obstructive bronchopneumopathy, and another 151 cases with chronic bronchitis without obstructive syndrome, over a median duration of 5 years. In the first lot of patients 37 cases of bronchopulmonary cancer have been recorded over this period of time, of which 35 in 333 male patients (10.5%), and 2 in 144 women (1.4%). The annual rate of bronchopulmonary cancer was of 1.9% in males, and of 0.2% in females. In the group of patients with simple bronchitis only 3 cases of bronchopulmonary cancer have been recorded, with an annual rate of 0.4%. In male patients with chronic obstructive bronchopneumopathy the annual risk of bronchopulmonary cancer is of 1900 0/0000, 60 times greater than in the general population, where it is of 30 0/0000. A comparative analysis of the three groups: chronic simple bronchitis, chronic obstructive bronchopneumopathy and chronic obstructive bronchopneumopathy complicated with bronchopulmonary cancer shows that in the last group there are several outstanding features: a more advanced median age, intensive (heavy) smoking, a more advanced degree of chronic alcohol intoxication, frequent professional exposure, genetic deficiencies of I.A.T., prolonged corticoid therapy in the antecedents, and a higher proportion of associated diseases. In general this group is more exposed to risk factors, and has a higher risk of alteration of the defense mechanisms of the organism. However, a more detailed analysis is necessary for detecting the factors involved in the genesis of bronchopulmonary cancer in patients with chronic obstructive bronchopneumopathies.