Currently endoscopic sinus surgery is the standard operation for rhinosinusitis and nasal polyps, including tumors near the anterior skull base. During the surgery, the iatrogenic penetration into the anterior skull base may be at risk. Therefore, information of the anterior skull base configuration is useful for the sinus surgeon.To determine the patterns and their prevalence of slopes of the ethmoidal roof assessed from the Computed Tomography (CTs) of paranasal sinuses.A descriptive study was conducted at Srinagarind Hospital, Faculty of Medicine, Thailand. CTs of paranasal sinuses from 150 patients aged 18 years and above who had attended at Srinagarind hospital between January 1, 2007 and December 31, 2011 were retrieved and reviewed. The slope patterns of the ethmoidal roof were determined by visual assessment of the scans, measuring the height between the ethmoidal roof and floor of nasal cavities in three sagittal planes: medio-sagittal, latero-sagittal, and mid-sagittal. CTs were performed by a Philips 128 slice scanner.The downslope anteriorly and upslope posteriorly of the ethmoidal roof was found to be commonest in medio-sagittal and mid-sagittal planes. Its prevalence in medio-sagittal and mid-sagittal planes was 80.3% (95% CI 75.5-84.4) and 52% (95% CI 46.4-57.6). The second common pattern in medio-sagittal and mid-sagittal planes was a downslope pattern with the prevalence of 13% (95% CI 9.7-17.3) and 39.3% (95% CI 34-44). The downslope pattern was found to be the most prevalent for the latero-sagittal plane, accounting for 50.7% (95% CI 45.0-56.3), followed by downslope anteriorly and upslope posteriorly of 40% (95% CI 34.6-45.6).This is the first study to describe the patterns of slopes of the ethmoidal roof (anterior skull base) which demonstrates that there are different patterns depending on the plane of sagittal view. Therefore, the sagittal view of the CT paranasal sinus in each plane should be interpreted before performing the operation to avoid iatrogenic intracranial complications.