Conclusions. The agger nasi cell, together with the postosuperior portion of the uncinate process, was the key that unlocked the frontal recess. Objectives. To investigate the anatomical interaction between the upper portion of the uncinate process and the agger nasi cell. Materials and methods. Twenty-one skeletal skulls (42 sides) were studied by spiral computed tomography (CT) and endoscopy, and one cadaver head ( 2 sides) was studied by collodion-embedded sectioning in the coronal plane. Results. The endoscopic view of the entrance of the middle meatus showed the middle part of the uncinate process and the middle part of the middle turbinate fused together as the axilla of the middle meatus. The middle portion of the uncinate process attached to the frontal process of the maxilla in all of the skeletal nasal cavities, as well as the lacrimal bone in 33 sides of the skeletal nasal cavities. On CT scans, the agger nasi cell was present in 38 sides of the skeletal nasal cavities. The agger nasi cell was medially, superiorly and inferiorly bounded by the uncinate process. The superior portion of the uncinate extended into the frontal recess and may insert into the lamina papyracea (33%), skull base (10%), middle turbinate, and a combination of these (57%).