Background: The pain of thoracotomy may be related to trauma to the intercostal nerves. Methods: This was a prospective randomized study of 160 patients. All patients had a functioning epidural, similar type and size thoracotomy, an intercostal muscle flap (ICM) harvested before rib spreading, inferior rib drilling, and postoperative pain management. In one group, the ICM was left intact distally and it dangled (D group); the ICM in the other was cut distally (C group). Pain was assessed using multiple pain scores. Outcomes assessed were qualitative and quantitative pain scores, number of ribs broken, spirometric values, analgesic use, and return to baseline activity for postoperative days 1 to 5 and weeks 2, 3, 4, 8, and 12. Results: The D group had 85 patients and the C group, 75. The groups had similar demographics, types of procedures, and histology. Intrahospital pain scores were similar; however, at postoperative weeks 3, 4, 8, and 12, the D group had significantly lower mean numeric pain scores and was using fewer analgesics (p < 0.05 for all). At 12 weeks, patients in the D group were more likely to have returned to baseline activity (p = 0.002). Conclusions: An ICM flap reduces pain. Harvesting and then leaving the ICM flap intact instead of cutting it before rib spreading further reduced thoracotomy pain. This technique, when added to rib drilling, leads to reduced pain on postoperative weeks 3 to 12, to quicker return to baseline activity, and lessens the need for analgesics. © 2008 The Society of Thoracic Surgeons.