Chronic maxillary atelectasis is a descriptive term that refers to a persistent decrease in the sinus volume of the maxilla from inward bowing of the antral walls. Case reports with comparable clinical presentations have appeared sporadically in the literature; however, this disorder has remained poorly defined. The purpose of this study is to provide a formal definition of this condition by the establishment of diagnostic and staging criteria. A 10-year case analysis identified 22 adults, and a review of the literature revealed another 25. The average age at presentation in our study was 38.3 years. Most patients were symptomatic, and some presented with diplopia and hypoglobus. Inward bowing of the antral wall(s) and persistent opacification on computed tomography made the diagnosis. Chronic maxillary atelectasis was separated into three stages according to the degree of wall deformation. While most patients were symptomatic, a past history of absent or mild symptoms referable to the nose and sinuses was encountered more often in those patients with osseous wall deformation (p = .041). Mild or absent symptoms at the time of diagnosis should not be considered a negative risk factor for the development of facial deformity, especially if the sinus has features consistent with complete pneumatization. A middle meatal antrostomy appears to relatively safely correct the sinus problem, while orbital floor reconstruction for hypoglobus, found in stage III of the disease, can be accomplished effectively via a transconjunctival approach using a combination of bone allograft and porous polyethylene sheets.