Maxillary sinus hypoplasia (MSH) is an uncommon clinical entity that represents a persistent decrease in sinus volume resulting from a centripetal retraction of the maxillary sinus walls. Pathophysiology of MSH is still controversial. Obstruction of the maxillary sinus ostium and negative pressure in the sinus has been mostly implicated in the development of MSH in most of the reported cases. We present a case of progressive unilateral MSH, which has an open ethmoid infundibulum and also presence of an open accessory ostium on the same side. Based on these findings, we propose that ventilatory dysfunction resulting from ostial obstruction alone cannot be accepted as the primary and sole etiologic factor in the pathogenesis of MSH.