We performed computed tomography (CT)-guided transbronchial diagnosis on 23 patients, using an ultrathin bronchoscope (external diameter, 2.8 mm) for small peripheral pulmonary lesions (< or = 2 cm). The mean size of the lesions was 1.4 x 1.1 cm. After examination using a conventional bronchoscope (external diameter, 6.3 mm), an ultrathin bronchoscope and a biopsy apparatus were advanced to the lesion under CT and X-ray fluoroscopic guidance. The location of the biopsy apparatus at each lesion was confirmed by thin-section CT, and a biopsy was performed. The ultrathin bronchoscope reached the 5th-11th bronchus (mean, 7.1 +/- 1.5th), at a point 3.6 +/- 0.9 bronchi peripheral to the site reached using a conventional bronchoscope. In 20 of the 23 patients, the biopsy apparatus could be guided to the lesion. The diagnosis rate, by disease, was 81.8% (9/11) for lung cancer, 66.7% (2/3) for metastatic lung cancer, and 77.8% (7/9) for inflammation; the overall rate being 78.3% (18/23). CT-guided transbronchial diagnosis using an ultrathin bronchoscope is useful for diagnosing small peripheral pulmonary lesions because the bronchoscope can be readily inserted into peripheral areas and guided to lesions, and the site of sample collection can be accurately determined.