The aim of this study was to investigate serial changes in bronchoalveolar lavage (BAL) cell profiles after human heart-lung transplantation and to assess the clinical value of BAL cytology in the differential diagnosis of complications in the transplanted lung. BAL was performed serially on 23 occasions on four patients. Elevated counts of neutrophils (4-48%) were observed in all preparations, with peak values in the early postoperative phase, in bacterial infections and in cytomegalovirus pneumonitis. In the last condition, BAL cytology also showed relative lymphocytosis (less than or equal to 50%) with high proportions (less than or equal to 50%) of HLA DR-positive T lymphocytes. No characteristic light microscopic pattern was observed in acute pulmonary rejection. However, scanning electron microscopy revealed elevated counts (greater than 5%) of "villous" macrophages in BAL obtained during or shortly after episodes of rejection. BAL cytology may be helpful in differentiating viral and bacterial infections, while scanning electron microscopy seems to be more suitable to the diagnosis of acute pulmonary rejection.