Proliferation of type II pneumocytes has been linked to a repair process during the early phase of acute lung injury, and it persists for a variable period. The mechanisms responsible for their dissolution and/or disappearance are not known, but we speculate that it may be partly due to apoptosis. Sections of lung tissue from patients with acute lung injury (n = 7) and chronic interstitial pneumonia (n = 14) were stained for detection of apoptotic cells via specific labeling of nuclear DNA fragmentation. Results were correlated with those of proliferating cell nuclear antigen (PCNA) staining for cell proliferation. Marked apoptosis of CD68-negative type II pneumocytes (30 to 80%) was detected in four of the seven (57%) cases of acute lung injury. In these cases, representing the resolution phase of acute lung injury, PCNA positivity in pneumocytes was extremely rare. In the 3 other cases in the acute/proliferative phase, apoptotic type II pneumocytes were rare whereas PCNA expression was quite evident in these cells. In chronic interstitial pneumonia, only rare type II pneumocytes (< 5%) exhibited apoptosis, and they showed variable staining for PCNA (up to 70%). We conclude that proliferation of type II pneumocytes occurs during the early phase of acute lung injury and is of variable extent and duration. In the resolution phase of acute lung injury, extensive apoptosis of type II pneumocytes is largely responsible for the disappearance of these cells. The time frame within which the apoptotic response occurs is variable and is likely to be dependent upon the specific etiology and extent of the injury. In chronic interstitial pneumonia, type II pneumocytes proliferate continuously, although to a much lesser degree than in the early phase of acute lung injury, and are minimally apoptotic.