Background: Little is known about the immunologic aspects and the pathogenesis of the paranasal sinus mucocele. Methods: The fluids of paranasal sinus mucoceles were obtained from 12 subjects. The concentration of macrophage migration inhibitory factor (MIF), interleukin 1β, tumor necrosis factor α, and regulated on activation normal T cell expressed and secreted (RANTES) were determined by enzyme-linked immunosorbent assay, and the levels of endotoxin were detected with kinetic Turbidimetric Assay. Results: MIF and endotoxin were detected in the fluid of all samples, whereas interleukin-1β and RANTES were detected in 1 and 3 subjects out of 12 samples. Tumor necrosis factor α was not detected in any of the samples. A significant positive correlation between the levels of MIF and the period with symptoms such as pain, swelling of face, and visual disturbance was observed. Conclusion: These findings suggest that MIF and endotoxin may play an important role in the pathogenesis of paranasal sinus mucocele. MIF may be an important factor causing the development and exacerbation of the disease.