BACKGROUND: Although thoracic epidural analgesia is a common practice in neuroaxial blockade for effective post-operative pain relief especially in major abdominal or thoracic surgery, difficult access to the thoracic epidural space is a frequent problem and can cause neurological complications like spinal cord injury and total spinal block. To minimize complications, we should to guess the distance for thoracic epidural space before this procedure. METHODS: One hundred fifty patients having preoperative upper abdominal computed tomography (CT) for diagnosis of their disease presented to major abdominal operation requiring mid-thoracic epidural analgesia for postoperative pain relief. The patient was placed in the sitting position and the levels of T7 and T8 spinous processes were identified. Using a paramedian approach with loss of resistance technique, when the insertion angles of Tuohy needle was measured by a protractor; inward angle (alpha) to the sagittal plane and downward angle (beta) to the transverse section of the spine. Entry of the needle into the epidural space, actual length (A) of the needle was marked and then measured with a ruler. Reviewing the abdominal CT films using the Picture Archiving and Communication System (PACS), the distance (B) from epidural space to skin on the transverse CT plane was measured at the corresponding to T7-T8 seemed to the level of the lowest scapular. The estimated length (Ac) of the skin to the epidural space was calculated by principle of trigonometry with alpha, beta and B. RESULTS: Mean (SD) age, height, weight, BMI were 56 (11) yr, 164 (6.9) cm, 61 (11.5) kg, and 37 (6.2) kg/m2, respectively. The A, B, Ac value and alpha, beta were 5.4 (0.77), 4.3 (0.76), 5.4 (0.85), 12 (3.4)o, 33 (9.6)o. There were significant correlation of both actual length of the needle and the estimated distance on CT film. Actual length of the needle tended to have 1.25 times longer than the estimated distance on CT film. There were also significances both A and weight, BMI, but not age, height. CONCLUSIONS: The distance from epidural space to skin measured on the transverse CT plane may be helpful as a guide for mid-thoracic epidural catheter insertion.