During 1986-1989, diagnostic fibre optic bronchoscopy (FOB) was performed in local anaesthesia in 1144 consecutive patients. Of these, 405 (35%) patients, median age 59 years, had transbronchial lung biopsy (TBB) performed under fluoroscopic guidance; 47 patients had a rebronchoscopy, i.e. in total 452 FOB were evaluated. The indication for TBB was localized pulmonary lesions in 279 (69%) patients, and diffuse pulmonary lesions in 126 (31%) patients. Localized lesions: TBB yielded a clinically relevant diagnosis in 55.2% of the patients. Of the 110 patients with malignancy, the overall diagnostic strength was 45.5%. Of the 159 patients with non-malignant lesions, 65.4% were diagnosed by TBB. The diagnostic yield increased with the number of biopsy specimens (< or = 4 biopsies, 52%; > 4 biopsies, 70%. P < 0.05). In 155 patients with well defined, circumscribed lesions, the diagnostic yield of TBB increased with the size of the lesion (< 31 mm, 47%; 31-60 mm, 54%; > 60 mm, 60%, P = 0.09), and decreased with the distance of the lesion from the main carina < 61 mm, 70%; 61-100 mm, 52%; > 100 mm, 40% P < 0.02). Diffuse lesions: TBB yielded a clinically relevant diagnosis in 66.7% of the patients. Of the 15 patients with malignancy, 73.3% were diagnosed by TBB. Of the 93 patients with non-malignant lesions, 78.5% were diagnosed by TBB. The diagnostic yield showed a trend to increase with the number of biopsy specimens (< or = 4 biopsies, 65%; > 4 biopsies, 71%, P = 0.11).(ABSTRACT TRUNCATED AT 250 WORDS)