The adult respiratory distress syndrome, bacterial pneumonia, cytomegalovirus pneumonitis, acute rejection, or a combination thereof were the primary causes of radiographic infiltrates or gas exchange abnormalities that occurred early after lung transplantation. The time of occurrence after transplantation, standard measures of clinical assessment as for nontransplant patients (i.e., vital signs, weight, white blood cell count, sputum, and cultures, etc.), bronchoalveolar lavage, and transbronchial lung biopsy were the primary tools used to analyze these situations. Bacterial pneumonia always occurred after postoperative day 2, acute rejection after postoperative day 5, and cytomegalovirus pneumonitis after postoperative day 16. Although cultures of bronchoalveolar lavage fluid were useful to detect pneumonia caused by bacteria, virus, and fungus, the types of cells recovered by bronchoalveolar lavage were not diagnostic of any type of disorder. Transbronchial lung biopsy was necessary to detect acute rejection and cytomegalovirus pneumonitis. Thus the cause of an early radiographic infiltrate or impairment of gas exchange was almost always reliably determined by using standard tools of clinical assessment, knowledge of the usual temporal sequence of the complications, and judicious use of bronchoalveolar lavage and transbronchial lung biopsy.