Autoimmune pancreatitis (AP) is one manifestation of a systemic, steroid-responsive disease with elevated serum IgG4 and characteristic histopathology, including increased IgG4-positive (+) plasma cells in the tissue. The histopathology of pulmonary IgG4 disease has not been well established. Six lung biopsies from patients with documented AP were studied, along with 12 additional cases showing similar pulmonary histopathology. For comparison, we examined Erdheim-Chester disease (n=3), pulmonary Sjögren syndrome (n=19), inflammatory myofibroblastic tumor (n=10), various inflammatory and interstitial lung disease (n=61), and nodal or extranodal Rosai-Dorfman disease (RD) in adults (n=8). All cases were stained for IgG4 and scored as 1, 2, and 3 as described in AP according to the following criteria: 0, <5 (per high power field); 1, 5 to10; 2, 11 to 30; and 3, >30. Five lung biopsies from AP patients showed IgG4 score of 3, and 1 had a score of 2. Consistent findings in lung biopsies of AP patients included endothelialitis of pulmonary vessels, active fibrosis, lymphangitic inflammatory infiltrates rich in plasma cells and histiocytes with or without nodule formation, and fibrinous pleuritis. Prominent lymphatic dilatation with histiocytes showing emperipolesis of lymphocytes was also seen. All 12 additional cases showing these histologic features also had the IgG4 score of 2 or 3. Among other conditions, an IgG4 score of 2 or 3 was seen in 6 of 8 RD, 4 of 10 inflammatory myofibroblastic tumors, and 8 of 61 inflammatory and interstitial lung disease, but in none of the rest. In conclusion, distinctive pulmonary histopathology was associated with increased IgG4+ cells in both AP patients and those unknown for AP status. The significance of increased IgG4+ cells in high proportion of RD cases merits further study as does overlap of RD and IgG4 disease.