Introduction: Mucopyocele is an unusual disease, that generally occurs in the frontal and ethmoidal sinus. Objective: To report a case of a large isolated mucopyocele of left maxillary sinus with facial asymmetry, and to confront the main clinical and surgical findings with medical literature. Case report: A 17 year-old male patient reported left hemifacial progressive edema, nasal obstruction and pain for the last 6 months. He had an important facial asymmetry, palatal bulging, and a severe septal deviation for the left nasal cavity. CT scan showed the presence of an extensive homogeneous mass on the left maxillary sinus, with liquid density. The endonasal approach confirmed the diagnosis of mucopyocele of the left maxillary sinus, probably caused by the septal deviation that blockage the medium meatus and the ostium of the sinus. Conclusion: Although the isolated mucopyocele in the maxillary sinus is rare, otolaryngologists should consider it in the differential diagnosis of expansive masses of the panasal sinuses. Endonasal approach is effective to treat mucopyoceles.