From March 1991 to December 1993, 30 patients underwent transbronchial biopsy (TBB) after lung transplantation (16 with a heart lung transplant, 7 with a single lung transplant, and 7 with a double lung transplant). The now standard TBB technique was used. Initially, TBB was performed only when clinically indicated, i.e. when there were sound reasons to suspect the existence of acute rejection (AR) or pulmonary infection. From 1992, all the patients were entered into a prospective study, the protocol of which called for serial "surveillance" TBB to be performed, in addition to those for clinical indications, 15 days, 2, 3, 6, 9 and 12 months after the transplant, and then annually. One hundred and twenty nine transbronchial biopsies were performed in 2.5 yrs. Of the 121 successful TBBs, 54 (45%) were positive, i.e. showed signs of acute rejection. Sixty six of 129 (51%) of the TBBs were performed because of clinical indications, 45 of them (68%) within the first 3 months following the transplant. The other 63 were surveillance biopsies. About two thirds of the clinically indicated TBBs and more than a quarter of the surveillance TBBs, yielding adequate samples, were positive for AR > or = A2 (mild rejection). The sensitivity and specificity of the method in detecting AR were 91 and 100%, respectively. The overall incidence of complications was 10.8% (pneumothorax in 9% of cases). There were no deaths correlated to the procedure. Our results confirm the decisive role of TBB in the diagnosis of acute lung rejection. The high incidence of mild acute rejection, and the occasional finding of moderate acute rejection in stable asymptomatic patients, support the use of surveillance TBB in the first 6 months.