Distinguishing lung rejection from infection in patients who have undergone heart-lung transplantation is difficult. Since bronchoalveolar lavage has been safely used to investigate other pulmonary conditions, its safety and the value of the cellular data obtained were evaluated on 44 occasions in ten heart-lung transplant recipients. This study established that bronchoalveolar lavage is safe after heart-lung transplantation, that it reliably diagnoses infection, and lastly that serial lavages are valuable in finding subclinical Pneumocystis carinii infections. The study failed to distinguish pulmonary infection from rejection. Nevertheless, the functional analysis of the cells obtained by bronchoalveolar lavage may yield information about the immune status of the allograft.