As pathologists, we are most concerned about overcalling reactive changes in the lung as carcinoma and the fact that malignant processes may be misinterpreted as benign processes in the lung is less well recognized. This review covers five such lesions. Well-differentiated adenocarcinomas, especially bronchioloalveolar carcinomas, are frequently undercalled, particularly in small biopsy and cytology specimens. In such cases, one must pay particular attention to the uniformity and monotony of the epithelium even though it may be extremely well differentiated. Spindle cell carcinomas may have necrosis and cavitation, interstitial growth, and a reactive fibroblastic reaction, and thus be mistaken as organizing inflammatory processes. Careful attention to the atypical cytological features, prominent vascular invasion, and getting immunohistochemical supports helps in recognizing them. Lymphoid lesions of the lung present a number of problems; small lymphocytic lymphomas and Hodgkin's disease are often misinterpreted as inflammatory processes. Intravascular lymphomatosis in the lung may be misinterpreted as an interstitial pneumonia if one does not appreciate the atypical lymphoid cells within the capillaries. The desmoplastic variant of sarcomatous mesothelioma may be extremely difficult to diagnose, because large portions of the tumors are composed of bland-appearing fibrous tissue. A case of desmoplastic mesothelioma presenting predominantly as a mediastinal mass is discussed, and problems in differential diagnosis are outlined. Angiosarcomas are rare tumors, but an appreciable percentage of them present as pulmonary metastases which may be interpreted as pulmonary hemorrhage or organizing infarction. The clinical and radiographic pattern, usually mimicking metastatic disease, and the fact that atypical spindle cells occlude small pulmonary arteries with surrounding alveolar hemorrhage are clues to the recognition of these lesions.