The objective of this study is to analyze the possible variations in size and shape of the AER in the ear affected by acquired cholesteatoma versus the healthy ear in the same patient. A total of 22 patients affected by acquired cholesteatoma were included in our study. A CT morphological evaluation of both ears (pathologic and non-pathologic) was made. Measures of the AER were done, on axial plane, parallel to incudomalleal axis for the deepest anterior-to-posterior (AP) diameter and perpendicular to this line for the maximum transverse (T) diameter, selecting the most inferior cut that showed the Cog in its entirety. A third superior-inferior (SI) measure was done, on coronal plane from the tegmen tympani to the cochleariform process. Comparisons between the mean of AP, T and SI in affected ears versus non-affected have been carried out using a paired t test. The AER measurement was considerably smaller in affected ears than in the non-affected ones. Mean AP +/- DS was 5.1 (1.46) versus 3.1 (0.90), P values <0.0001. Mean T +/- DS was 4.1 (0.74) versus 3.2 (0.74), P values <0.0014. Mean SI +/- DS was 4.0 (1.01) versus 2.0 (0.82), P values <0.0001. In conclusion, based on our results, the AER in an affected ear seems smaller than in a non-affected one. Whether a hypovolumetric AER could be a congenital morphological condition predisposing cholesteatoma despite adequate aeration of the epitympanic compartment, on the contrary the presence of membranous and/or ligamentous folds could exclude the AER from the posterior epitympanic space and from the protympanum, predisposing it for attical dysventilation, should be clarified in further studies.