Exclusive discohesive tumor cells within intra-alveolar spaces is a rare pattern of involvement of primary and metastatic lung tumors. In the absence of a tumoral mass, this pattern closely mimics desquamative interstitial pneumonia (DIP) clinically, radiologically, and histologically. However, a secondary DIP pattern may be seen adjacent to a tumor mass not infrequently. Here, we describe a case of a 64-year-old woman status post bilateral lung transplantation, who was radiologically thought to have an interstitial lung disease. The autopsy lung specimen revealed extensive involvement by intra-alveolar adenocarcinoma, with a cytomorphology mimicking alveolar macrophages as seen in DIP in the absence of a tumoral mass. The presence of subtle clustering with signet ring cell features and rare micropapillae were an important clue to the diagnosis. The tumor cells were positive for CK7, napsin-A, and TTF-1, and negative for CD68. This case represents an unusual variant of pulmonary adenocarcinoma with a pure intra-alveolar pattern of involvement, mimicking DIP. To the best of our knowledge, excluding biopsy specimens, only one similar case has been reported in the literature and none in the posttransplant setting.