Pseudotumor is a space occupying inflammatory lesion which mimics a true neoplastic process in its clinical symptoms and radiologic findings. Pseudotumor of maxillary sinus is rare and the etiology is unknown. Because of bony destruction of maxillary sinus, it is difficult to distinguish this lesion from malignant tumor, fungal sinusitis, or granulomatous diseases such as tuberculosis, syphilis, sarcoidosis, and Wegener's granulomatosis. We experienced a patient with an unusual pseudotumor of the left maxillary sinus which was suspected to be a maxillary cancer in its clinical symptoms and radiologic findings. It was removed surgically by left lateral rhinotomy, using a Caldwell-Luc's approach under general anesthesia. The histopathologic findings showed dilatation of blood vessels, infiltration of inflammatory cells and degeneration of cells without any evidence of malignant changes.