AimsConcomitant occurrence of Mikulicz's disease (MD) and immunoglobulin (Ig)G4-related chronic rhinosinusitis (IgG4-related CRS) is extremely rare. We evaluated the clinicopathological features of MD patients with concomitant IgG4-related CRS (CRS-MD). Methods and resultsTwelve CRS-MD patients were evaluated clinically and biopsy samples were taken from the lacrimal/salivary glands (n=12) and nasal mucosa (n=7) for assessment of IgG4-positive cells, using immunohistochemical techniques. Similarly, nine MD patients and 10 patients with common CRS were evaluated as controls. CRS-MD patients had higher serum IgG and IgG4 concentrations than MD patients (P<0.05 for both). Lymphoplasmacytic infiltration, lymphoid follicle formation and sclerosis was prominent in the lacrimal/salivary glands in both groups; however, the magnitude of IgG4-positive plasma cells infiltration in the CRS-MD group was significantly higher compared to the MD group (P=0.004). Similarly, evaluation of nasal mucosa revealed greater lymphocyte, plasma cell and eosinophil infiltration and lymphoid follicle formation, together with significantly higher IgG4-positive plasma cell infiltration in the CRS-MD group compared to the common CRS group (P=0.004). ConclusionsConcomitant MD and IgG4-related CRS were characterized by a combination of IgG4-positive plasma cells infiltration in the lacrimal/salivary glands and the nasal mucosa and increased serum IgG4.