To study the diagnostic yield of transbronchial biopsy (TBB) in lung diseases of different ethiology, as well as to examine several factors implicated in diagnostic yield and complications of TBB. Retrospective study of a series of 172 patients (98 women and 74 men) undergoing TBB. Clinical variables, x-ray and CT patterns and technical factors related to TBB were analyzed in relation to diagnostic yield and definitive diagnosis. We recorded the main complications and studied the possible mechanisms implicated in their appearance. The results of TBB were classified as follows: 1) diagnostic TBB (42.8%); 2) nonspecific TBB (21.1%); absence of parenchyma (NP) (9.7%); 4) normal parenchyma (23.4%); 5) incorrect diagnosis (2.9%). Overall yield was 43.6% but rose to 52% when NP and no infiltrative pneumopathies were excluded. The main findings were infections (23.4%), neoplasms (19.4%), sarcoidosis (14.2%), idiopathic pulmonary fibrosis (IPF) (17.7%); and other (23.4%). Significant differences in diagnosis were found for age, sex, time of evolution prior to TBB, and x-ray and CT patterns. Complications and pneumothorax appeared more frequently in IPF, neoplasms and infections, although the differences were non significant. Diagnostic value depended mainly on type of disease and ranged from 60% for sarcoidosis to 24% for IPF (p < 0.05), with no differences related to age, sex, presence of immunosuppression, number of TBB or tolerance to exploration. Yield was higher, however, for patients with no loss of radiographic pulmonary volume (56.3% versus 37.5%) and with absence of a CT reticular pattern (44.7% versus 27.3%). Type of opacites also influenced yield (53.4% for ground glass versus 26.7% for reticular patterns), as did distribution of lesions (8.4% for peripheral patterns versus 48.3% for diffuse patterns and 50% for peribronchio-vascular patterns). The site of TBB was the only factor showing clearly significant differences in yield (100% in mild lobe versus 29.5% in upper lobes). Tolerance was poorer in older patients (63.5 +/- 10 versus 52.2 +/- 17 years) and with greater function abnormalities (%FEV1: 48.2 +/- 16.8 versus 70.2 +/- 17.1). Poor tolerance of exploration was associated with the appearance of complications (16.6% vs 6.3) and pneumothorax (25% versus 6.8%). The diagnostic yield of TBB was mainly influenced by the type of disease studied. The various imaging techniques were fundamental for establishing preliminary diagnoses and degree of evolution of a specific cases. In our series, tolerance of the technique was a reliable predictor of complications and the presence of post-biops pneumothorax.