Seven cases of single lung transplantation are reported. The recipients were all below 60 years of age and severely disabled with end-stage lung disease. Transplantation was performed according to ABO blood group compatibility and negative lymphocytotoxic cross-match between donor and recipient irrespective of HLA mismatch. Recipients' diagnoses were sarcoidosis (3), alfa-1 antitrypsin deficiency (3), and idiopathic emphysema (1). Mean recipient age was 48 +/- 2.4 years (range 45-52). Donor age was 29.7 +/- 5.6 years (range 16-49). The immunosuppressive regimen included cyclosporin A, azathioprine, steroids and rabbit antithymocyte globulin. Excellent graft function was achieved. Six patients survived the postoperative period and are alive 4-18 months posttransplant. One patient died after the operation due to pneumonia with respiratory distress syndrome. Graft function was also monitored by transbronchial biopsy, and 57 biopsy procedures were performed without fatal complications. Acute cellular rejection was seen in 16 biopsy specimens from 5 recipients (grade 1 and 2 rejection in 14, grade 3 rejection in 2). Neither severe rejection with septal necrosis (grade 4) nor obliterative bronchiolitis was seen. The rejection rate was 0.03 episodes per patient/month. In contrast to other reports, episodes of cellular rejection occurred throughout the observation period, and were not mainly limited to the first 4 months posttransplant. Graft vascular occlusive disease or chronic vascular rejection was found in 6 biopsy specimens from one recipient. Five patients experienced 7 episodes of cytomegalovirus infection. The cytomegalovirus infection rate was 0.01 episodes per patient/month. The incidence of infection was significantly lower compared to previous studies of rejection in other lung graft combinations. Both infections and rejection episodes may contribute to the development of obliterative bronchiolitis. Almost one third of the specimens (30%) showed lymphocytic bronchitis without perivascular inflammation. The absence of perivascular infiltrates and exclusion of infectious agents leaves in question the aetiology of this inflammation. The lymphocytic bronchitis could be ischaemic, related to aspiration, or represent recurrent sarcoidosis, or, in fact, express bronchial rejection. All biopsy specimens regarded as rejection with cellular infiltrates in the lung parenchyma also showed a lymphocytic bronchitis. The impact of HLA mismatch on cellular and vascular rejection is unclear. Transbronchial biopsy is a reasonably safe and reliable method in the diagnosis of rejection and infection in single lung transplantation.