ive] To assess the diagnostic value of fiberbronchoscopy for right middle lobe syndrome (RMLS) and to explore the etiology of RMLS. [Methods] Fiberbronchoscopy was carried out in 42 casesof RMLS diagnosed by chest X-ray or CT. [Results] Patients with RMLS due to non-specific inflamma tion accounted for 57.1 % , lung cancer 26.2%and pulmonary tuberculosis 11.9%. Congestion, edema, stenosis, neoplasm, purulent secretion, and mucosa thickening were the main manifestations under fiber-bronchoscope. The diagnostic accordance rate of fiberbronchoscopy for lung cancer, non-specific inflam mation and pulmonary tuberculosis was 90.9% , 87.5% and 60% respectively. No significant differences of the incidences of non-specific inflammation and lung cancer were found in the teenage, the adult and the aged. [Conclusion] Fiberbronchoscopy is an important method of the diagnosis of RMLS. Lung can cer, non-specific inflammation and pulmonary tuberculosis are the common pathogenic factors of RMLS. Male cases over 40 should take fiberbronchoscopy to exclude the possibility of lung cancer.