In an eleven year period at Muirdale Sanatorium, 47 cases of primary bronchogenic carcinoma were observed. These patients were sent in as cases of pulmonary tuberculosis. Thirty-five of the 47 cases were proved by postmortem examination, biopsy and one by the presence of tumor cells in the pleural fluid. Twelve cases were diagnosed on the basis of clinical, roentgenologic and bronchoscopic findings. Two cases had both pulmonary tuberculosis and primary bronchogenic carcinoma. The average age of our patients was 55.4 years. Our youngest patient was 40 years old and the oldest was 73 years of age. The ratio of male to female was 3 to 1. Of the proved cases, 14 were squamous cell type, 14 undifferentiated-cell carcinoma, 3 adenocarcinoma and one alveolar cell carcinoma. Two cases could not be classified histologically. The importance of early recognition of primary bronchogenic carcinoma is emphasized by the fact that only four of the 47 patients were considered operable at the time of diagnosis. The interval from diagnosis to death was only 3.8 months, while a period of 10.7 months elapsed from the onset of symptoms to diagnosis. The patient's delay in seeking medical advice was 6.4 months and the physician's delay in establishing the diagnosis was 4.3 months. The clinical picture of primary bronchogenic carcinoma is mainly respiratory in character; the prominent symptoms are cough, chest pain, hemoptysis, dyspnea. Weight loss was a constant finding. Fever and leucocytosis depend upon the presence of secondary pneumonitis. No significant anemia was noted in any of our cases. The importance of tuberculin testing adults is re-emphasized. Nine of our patients had negative tuberculin test and should never have been considered as tuberculous. The differential cell count of the pleural fluid revealed a marked lymphocytosis in three cases. Lymphocytosis is considered typical of tuberculous pleurisy with effusion and in these cases could, mislead one to an erroneous conclusion. Clubbing of the fingers is not an unusual finding in bronchogenic carcinoma. The importance of roentgenograms, bronchoscopy, cytological examination of sputum and bronchial secretions and thoracotomy as diagnostic procedures are discussed.