Introduction: to perform a successful endoscopic frontal sinus surgery, prevent complications, and lower the recurrence risk, it is essential to understand the anatomy of the frontal sinus (FS) and frontal recess cells with many variations in frontal sinus drainage (FSD). Aim: preoperative assessment of the FSD in three levels in order to find prognostic factors in the decision process regarding the kind and the extent of surgery when required. Method: three FSD levels were assessed by computed tomography in two dimensions; antero-posteriorly and laterally in 100 consecutive patients with chronic sinusitis symptoms. The first level represents the proper drainage of FS. The second level is the drainage of FS without the effect of the frontoethmoidal cells. The third level is the maximum drainage that can be achieved in a single FS. The relation of FSD levels to FS and frontoethmoidal cells pathology were assisted. Results: within 100 patients (200 sides, 186 FSs), for the proper FSD, antero-posterior (AP) length was 5.94±3.42 mm in opaque FS and 5.32±2.87 mm in clear FS and its lateral length was 3.04± 1.6 mm in opaque FS and 2.30±1.25 mm in clear FS. For the functional FSD, AP length was 8.97±2.7 mm in opaque FS and 8.05±2.7 mm in clear FS and its lateral length was 7.51±1.69 mm in opaque FS and 7.58±1.75 mm in clear FS. In the anatomical FSD, AP length was 11.25±3.07 mm in opaque FS and 10.01±2.87 mm in clear FS and its lateral length was 11.1±2.6 mm in opaque FS and 10.95±1.7 mm in clear FS. Conclusion: this study offers essential data for preoperative assessment in order to improve surgeons’ awareness of thefrontoethmoidal region for optimal safe EFSS with lower incidence of complications and recurrences.