No consistent identifiable risk factors have been reported. Pathologically, inflammatory pseudotumors are thought to originate from an organizing pneumonia. Symptoms consistent with a preceding infection occur in approximately one third of patients, but there is no clear information on time course after the infection or correlation of location between the infection and the subsequent tumor. Several reports linking infection to pseudotumors have included viral, bacterial, rickettsial, and fungal etiologies. Pseudotumors are categorized into three subtypes: organizing pneumonia, fibrous histiocytoma, and lymphoplasmacytic. Several features are common to all subtypes and include proliferation of spindle-shaped fibroblasts, granulomatous inflammation, lymphoid hyperplasia, fibrosis, and invasion of lymphocytes. Significant overlap among the three subtypes occurs. The organizing pneumonia subtype occurs with the greatest frequency and is characterized by intra-alveolar lymphohistiocytic inflammation and fibrosis with preserved alveolar architecture. The fibrous histiocytoma subtype is characterized by a proliferation of spindle cells in a storiform pattern with inflammatory cells at the periphery. In the lymphoplasmacytic subtype, plasma cells and lymphocytes predominate often with lymphoid aggregates. This is the least common of the subtypes. No distinct clinical or radiographic features have been described to differentiate the three subtypes.