Twenty-two patients undergoing thoracotomy were enrolled in a controlled, randomized trial to compare the effects of thoracic (group T) or lumbar (group L) epidural morphine in relieving postoperative pain. Epidural morphine was given as the patient first complained of pain in the recovery room. The effect of epidural morphine on pain relief was assessed by visual analogue scale and subjective grade. All patients were investigated 0, 10, 20, 30, 40, 50, 60, 75, 90, 105 and 120 minutes after a single bolus injection of epidural morphine. In assessment of pain relief by visual analogue scale, the difference between two groups was statistically significant (p < 0.05), evaluated 20, 30 and 40 minutes following treatment. There was no significant difference between two groups with regard to the effect of morphine as assessed by subjective grading. We concluded that thoracic epidural morphine took effect faster than lumbar epidural morphine. However, the effects of both kept similar since 50 minutes after morphine injection.