Caldwell-Luc operation is one of the most frequently performed operations for the treatment of maxillary antral disease with advantages of easy technique and good visualization, but complications are developed in 10-40% of the cases. Among them, postoperative maxillary cyst is sometimes troublesome complication which is more frequently developed in Orientals than Westerns. The pathogenesis is unknown but several suggestions have been postulated. So, by measuring the postoperative maxillary antral walls using computed tomography and comparing them with normal and chronic sinusitis maxillary antra, we have investigated the pathogenesis of the postoperative maxillary cyst. There were total 69 cases of maxillary sinus computed tomograms consited of 29 cases of normal group, 16 cases of chronic sinusitis group and 24 cases of post-Caldwell-Luc operation group. The average period after the surgery was 25.3±10.4 years and statistical significance was evaluated by T-test separately with male and female. Six parameters were evaluated in coronal and axial scans for the assessment of the postoperative changes of maxillary antral walls. There was no significant changes between normal and chronic sinusitis group. But statistically significant changes were noted between normal and postoperative group. The postoperative group had a tendency for becoming hypoplastic antrum which means that maxillary antral wall is contracted centripetally after Caldwell-Luc operation. With retained mucosa in the antrum, accumulation of body fluid and blood and formation of granulation tissues in the antrum, a hypoplastic antrum by the centropetally contracted antral walls may contribute for the formation of postoperative maxillary cyst is suggested.