Purpose : Trans-sphenoidal-sinus approach with Hardy's endoscope is the standard technique for pituitary tumor. However, this approach has difficulty with limited operation field and requires modification according to 1) shape of nasal septum, 2) structure of the sphenoidal-sinus, and 3) tumor size. Preoperative 3D-CT may enable simulation of the approach with accurate nasal septum and sphenoidal-sinus morphology and tumor size. Methods and Materials : Preoperative 3D-CT was performed in 8 patients with pituitary tumor. Based on the 3D-CT images, 1) shape of nasal septum, 2) structure of the sphenoidal-sinus, and 3) tumor size viewed from trans-sphenoidal-sinus approach was evaluated and compared with the actual findings obtained by surgery. Results : 1) Shape of the nasal septum (no deviation, deviated to the left or right) by preoperative 3D-CT was identical to the findings by surgery. 2) Structural type (sellar, presellar, or conchal) determined by 3D-CT agreed well with the type determined by surgery. 3) Tumor size by 3D-CT was also identical with the actual size measured at the time of surgery. Conclusions : Preoperative 3D-CT enables simulation of trans-sphenoidal-sinus approach with accurate nasal septum and sphenoidal-sinus morphology and tumor size.