OBJECTIVE To compare the results of 0.2 mg and 0.3 mg of spinal morphine in patients with post-thoracotomy pain. METHODS A prospective, randomized, double-blind study. METHODS A university hospital and a tertiary referral center. METHODS Forty patients undergoing thoracotomy for lung resection. METHODS Spinal morphine: 0.2 mg or 0.3 mg. RESULTS Patients were randomly allocated to receive either 0.2 mg or 0.3 mg of spinal morphine for post-thoracotomy pain prior to general anesthesia. Cumulative 24- and 48-hour intravenous patient-controlled analgesia meperidine consumption and numeric rating scale score for pain were recorded. The severity score (4-point scale) of sedation, nausea, vomiting, and pruritus was assessed for interval during the postoperative period. The times to first drinking/eating/sitting/walking were recorded. There was no statistically significant difference in 24-hour postoperative meperidine usages between the 2 groups, which were 110 (interquartile range 90-180) and 95 (interquartile range 57.5-175) mg, respectively. The median pain free times, time to first drinking, eating, sitting, and walking showed no differences between the 2 groups. The numeric rating scale score for pain and incidences of side effects were not different between the 2 groups. One patient who received 0.3 mg of spinal morphine developed respiratory acidosis with a respiratory rate of 10 per minute at 44 minutes after extubation. CONCLUSIONS In patients who underwent elective thoracotomy, 0.2 mg of spinal morphine was as effective as 0.3 mg. Increasing the dosage of spinal morphine does not decrease postoperative meperidine consumption, but may lead to respiratory depression in rare cases.