This study was designed to describe the anatomic features of the frontal recess by transnasal endoscopy (nasoscope), to analyze its implications in endoscopic frontal sinus surgery and to discuss the issues that may be experienced during such operations.The patients included in this analysis were adults with chronic sinusitis or nasal polyp (n = 301, 562 sides) hospitalized in our hospital from August 1998 to April 2001. Chronic frontal sinusitis was confirmed in 280 cases (479 sides) by coronal and axial CT scan. Patients with a previous surgical history were excluded from the analysis. The surgical outcomes of these patients and CT imaging data were retrospectively reviewed and analyzed. The drainage pattern of the frontal sinus was identified based on CT scans preoperatively. Endoscopic frontal sinus surgery was performed in 250 cases (421 sides). The anatomic features of the frontal recess under nasoscope were classified into 2 types.The coronal CT results confirmed the position of the frontal sinus ostium between the uncinate process and the middle turbinate in 203 sides (48.2%) of all operated patients and the ostium was found to be located between the uncinate process and the lamina papyracea in 218 sides (51.8%). According to the location of the frontal sinus ostium, we grouped the anatomic features of the frontal recess into 2 types. Type I was documented in 203 sides (48.2%) and type II was seen in 218 sides (51.8%).The anatomic features of the frontal recess under nasoscope and their classification are very important and helpful for endoscopic frontal sinsus surgery. The upper part of the uncinate process is a dependable anatomic landmark for the localization of the frontal sinus ostium in CT scan and endoscopic frontal sinus surgery.