PURPOSE: To determine the CT factors which predict positive results of bronchoscopic biopsy in cases of lung cancer with CT-bronchus sign. MATERIALS AND METHODS: In 30 patients who on CT showed a lung mass with CT-bronchus sign and who had undergone bronchoscopic biopsy, lung cancer was confirmed histopathologically and/or clinically. The CT findings were evaluated for the location, nature and size of the mass, and the type of CT-bronchus sign, and the diagnostic rate of bronchoscopic biopsy and of CT findings was compared. RESULTS: Seventeen of 30 patients (56.7%) were diagnosed by bronchoscopic biopsy and their diagnostic rates according to the location were as follows: 100% (10/10) in cases involving the lobar bronchus; 60% (6/10) in cases involving the proximal segmental bronchus and 10% (1/10) in cases involving the distal segmental bronchus. In 20 cases of peripheral lung cancer, 16.7% (1/6) of masses with less than 3cm in diameter, 44.4% (4/9) of masses with more than 3cm and less than 6cm, and 40.0% (2/5) of masses with more than 6cm were diagnosed bronchoscopically. In addition, 57.1% (4/7) of cases with abrupt bronchial obstruction, 33.3% (3/9) with a patent bronchus within the mass, 0% (0/3) with bronchial displacement or a marginally located bronchus and 0% (0/1) with tapered bronchial obstruction were diagnosed on bronchoscopic biopsy. One of two cases with perilesional lymphangitic spread and two of four cases with a large cavity were diagnosed bronchoscopically. CONCLUSION: In cases of lung cancer, bronchoscopic biopsy is a useful initial diagnostic method where the mass is located in 1cm proximal to segmental bronchial bifurcation and is more than 3cm in diameter, there is CT-bronchus sign with abrupt bronchial obstruction or a patent bronchus within the mass, and associated perilesional lymphangitic spread or large air-cavity. In most cases where there is peripheral lung mass less than 3cm in diameter, however, bronchoscopic biopsy alone is not adequate, and the use of a further diagnostic modality is required.