Malignant tumors in the lung (both primary and metastatic) rarely may be associated with markedly discohesive tumor cells, resulting in airspace filling reminiscent of "desquamative interstitial pneumonia" (DIP) on histopathology evaluation. A peculiar aspect of this growth pattern is the relatively bland appearance of the tumor cells, in many cases simulating benign alveolar macrophages at scanning magnification. We searched the Charles Carrington Memorial consultation files in the Department of Laboratory Medicine and Pathology at Mayo Clinic Arizona for instances of malignant tumors in lung simulating DIP, from 1992 to 2011. We identified 7 cases involving transbronchial biopsies, needle core samples, or resected lung specimens. Clinical, histopathologic, and immunohistochemical analyses of these 7 patients were performed, including detailed morphometric analysis of the individual tumor cells using calibrated measurement tools on digital images. We compared the results with those of a control group of 4 patients with benign DIP-macrophage reactions in smoking-related lung disease. The study group comprised 5 male and 2 female patients, 48 to 86 years in age (median: 67 y). The radiologic findings included lobar consolidation, localized ground-glass opacities, and 1 or more nodules. None of the patients had typical bilateral infiltrates of DIP. Microscopically, the lung parenchyma was dominated by the presence of prominent tumor cells filling alveolar spaces. Four patients had primary lung carcinoma (adenocarcinoma), whereas 3 had metastases from other sites, including a melanoma. Immunohistochemical staining studies were performed on 6 of 7 cases to establish the diagnosis. Nuclear diameter, cytoplasmic diameter, and nuclear/cytoplasmic (N/C) ratios in patient and control groups were compared using the Wilcoxon rank sum test. No significant difference in the diameters of nucleus and cytoplasm between cases and control groups (P=0.3447 and 0.7055, respectively) was seen, and only a marginally significant difference in N/C ratios (P=0.0890) was seen. A more complex analysis, generalized estimating equation analysis, showed a significant difference in N/C ratio between the 2 groups (P=0.0278). A "DIP-growth pattern" of malignant tumors in the lung is presented. Although the N/C ratio differences approached statistical significance when compared with controls, the key to diagnosis is the recognition of the malignant cytology of the tumor nuclei. Immunohistochemical studies (keratin or other markers) are helpful in establishing an accurate diagnosis in this setting.