Results obtained in the cytologic study of sputa from 630 patients are presented. There were 251 cases of bronchogenic carcinoma; diagnosis through sputum examination was possible in 57.4% of the patients. Abnormal cells were detected in an additional 24.3%. Sputum examination proved to be a valuable complement by establishing the correct diagnosis when other methods failed. Cancer cells were unequivocally identified in 45.8% of cases with normal bronchoscopic examination and 52.4% of cases in which bronchial biopsy did not include malignant tissue. The same proportion of cases with various tumor types was obtained by cytologic and histologic study. One of the methods often showed a higher degree of cellular differentiation than the other. The number of cases with undifferentiated cancer or unclassified tumors was reduced when the information concerning cell differentiation available through both methods was used. In this manner, excluding oat cell carcinomas, only 7.6% of cases of bronchogenic carcinoma did not show any cellular differentiation. Wider use of the information provided by simultaneous evaluation of cytologic smears and tissue sections is recommended to achieve a more accurate appraisal of tumor type.