Apoptosis of type II pneumocytes has been identified in diffuse alveolar damage (DAD), is associated with p53 and WAF1 expression, and may be of pathogenetic importance. BAX, a homologue of BCL-2, is induced by p53 and is a promoter of apoptosis. The proapoptotic effect of BAX is negatively regulated by its binding with BCL-2. In this study, we sought to investigate that role of BAX and BCL-2 in DAD. We hypothesized that alterations in BAX and BCL-2 expression may be important in determining the susceptibility of type II pneumocytes and interstitial cells to apoptosis. Twenty-eight cases of DAD and 16 control cases (i.e., lung tissues adjacent to resected tumors) were retrieved from the files of the University of Utah and the Armed Forces Institute of Pathology. Immunohistochemical stains were performed with antigen retrieval by microwave using antibodies recognizing BAX and BCL-2. The percentage of positively staining pneumocytes and interstitial cells was estimated in each case to the nearest 10%. BAX expression was markedly increased in pneumocytes and interstitial cells in DAD compared with control lung tissues. In DAD, BAX was identified on an average of 80% of alveolar pneumocytes (range 30 to 100%) and 70% of interstitial cells (range 20 to 90%). In control lungs, BAX was identified on an average of 10% of pneumocytes (range 0 to 20%) but not in interstitial cells. Focal BCL-2 staining was identified in interstitial myofibroblasts in 7 of 25 cases of DAD but was only identified in bronchiolar epithelium of control lungs. These results suggest that the induction of BAX in DAD may enhance the susceptibility of alveolar epithelial cells to apoptosis, whereas BCL-2 expression may contribute to the absence of apoptosis in interstitial myofibroblasts. Expression of BCL-2 in interstitial myofibroblasts may contribute to the development of pulmonary fibrosis in some patients.