A diagnosis of carcinoma based on cytologic examination of sputum or bronchial secretions has been made on 300 patients. This total has been divided into 2 groups, the first 150 cases representing a period of investigation with the method and the second 150 cases representing a period of evaluation of the method as a routine procedure in the diagnosis of thoracic diseases. Proved false positive results in 3 cases in the first series and in 1 in the second series were reported. In a group of 588 patients with miscellaneous diseases of the thorax, on whom a routine cytologic examination of sputum or bronchial secretions was carried but, 147 cases of proved or suspected bronchogenic carcinoma occurred. Of this number, in 100, or 68% of the total, the lesions were detected by a cytologic examination of sputum or bronchial secretions.