The Bronchogenic Carcinoma Co-operative Group analyzed the epidemiologic data of 240 cases of bronchogenic carcinoma in a prospective study. When risk factors (exposure to potencial pathogenic substances) were studied, a relationship was noted between the incidence at an early age and the presence of nondifferentiated small cell carcinoma. The number of years of exposure to tobacco had a direct effect on the curative value of the thoracotomy and the survival rate. Forty years of smoking appeared to be borderline for a poor prognosis. Localization of the tumor by bronchoscopy showed a frequency in the left main bronchus (18 percent) more than twice than in the right (8 percent). In 25 percent of the cases the tumor was more peripheral than the subsegmental situation according to the bronchofiberoptic examination. Evaluation of clinical data only showed a lack of correlation between the different symptoms and the various means of evaluation and the prognosis of the disease. For example, a patient with hemoptysis has a statistically higher probability of having an epidermoid tumor and a greater possibility of a successful curative thoracotomy (23 percent as opposed to 17 percent for the rest). The group of incidental cases (9 percent of the total) was analyzed. The degree of resectability was higher as compared with the rest (62 percent against 34 percent) as well as the absence of nondifferentiated small cell type carcinomas.