A group of 34 heart-lung transplant patients were studied with serial pulmonary function measurements, chest radiographs, and transbronchial biopsies from the time of surgery. These investigations were carried out routinely at 3 and 6 months and then annually after transplantation as well as on clinical suspicion of acute lung rejection or infection. A total of 61 transbronchial biopsies and concurrent lung function and chest radiographs were obtained. Of the biopsies, 30 (49.2%) showed histologic evidence of lung rejection, 12 (19.7%) demonstrated various opportunistic infections, and 19 (31.1%) were normal. Compared to during episodes of normal biopsies, FEV1 decreased significantly with lung rejection (p less than 0.001) and with infection (p less than 0.01). Vital capacity (VC) and DLCO also fell with these acute lung complications. Using histologic diagnosis as a standard, lung function testing had a sensitivity of 86% in detecting lung rejection in the first 3 months postoperation and 75% in the subsequent period. Its sensitivity for detecting lung infection was 75%. Although not distinguishing between these two complications, lung function had a specificity of 84% for detecting occurrence of an acute lung complication. Chest radiographs, although of similar sensitivity in the first 3 months postsurgery, had a sensitivity of only 19% for rejection in subsequent months and 58% for infection. Its specificity was 100%. Lung function testing changes in a predictable fashion with lung rejection and infection, offers an improvement over chest radiographs, and provides a quantitative measurement to aid the decision of when to undertake transbronchial lung biopsy.