Misdiagnosis of maxillary sinus hypophasia usually as sinus infection, sometimes as neoplasm, can lead to unnecessary and difficult surgical explorations. Associated anatomical abnormalities, e.g., caudal displacement of the ipsilateral frontal lobe of the brain or central position in the maxilla of the infraorbital nerve may create unexpected surgical hazards. Associated orbital enlargement can lead to diagnostic confusion in the investigation of headache, especially if the superior orbital fissures show marked asymmetry suggesting erosion. Projection of the fissure into the antrum in Waters view can simulate trabeculation of the sinus or fracture of the inferior orbital rim. Radiologic examination of 500 patients without intracranial or intraorbital lesions revealed maxillary sinus hypoplasia in 36 cases (7.2%) and in half the hypoplasia was unilateral (sinus asymmetry); aplasia was not encountered. Fissure asymmetry was present in 30 cases (6%), being present in 3.66% of patients with normally developed sinuses and in 36.1% of the hypoplasia patients. The appearances and measurements of the fissure are presented and examples of marked normal fissure asymmetry are demonstrated. Maxillary sinus hypoplasia is classified in this series as grade I-mild hypoplasia with limited inferolateral expansion (4 cases) and grade II in which there is also a curved orbital floor and lateral displacement of the adjacent nasal wall (32 cases).