To investigate the long-term histologic consequences of combined heart and lung transplantation, heart and lung biopsies were obtained from six rhesus monkeys; two had undergone heart-lung autotransplantation 3.5 and 4.5 years previously, two were the recipients of heart-lung allografts 4.1 and 4.5 years previously, and the results were compared with two normal control animals. Cyclosporine had been used as maintenance immunosuppression in the allograft group. The heart and lung biopsies in the autograft animals were essentially normal. Dense adhesions were noted in the allografts, adn in one the visceral pleura was grossly thickened. Cardiac biopsies in the allografts were unimpressive, with a normal myocardium in one, and minimal interstitial fibrosis in the other. Intimal hyperplasia was present in the pulmonary arterioles of one of the allografted animals. Focal scarring was present in the lung of one allograft recipient, and the other animal showed severe thickening and fibrosis of the alveolar septae, as well as marked interstitial fibrosis such that large areas of the specimen were replaced by connective tissue. Histologic abnormalities in the allografted lungs correlated with the abnormal hemodynamics in these animals reported in a previous study. It is suggested that the histologic appearances in the lung are a consequence of chronic rejection, and that these findings may become a significant problem in human heart-lung transplant recipients.