Background The surgical outcomes of decompression for thoracic spinal stenosis (TSS) are unfavorable.The purpose of this study was to determine the efficacy of intraoperative ultrasonography during “cave-in” 360° circumferential decompression surgery in patients with TSS.Methods Thirteen patients with TSS underwent “cave-in” 360° circumferential decompression surgery between May 2010 and November 2010.Intraoperative ultrasonography was used after removal of the posterior wall of thoracic spinal canal to assess the morphologic restoration of the spinal cord and the anterior surface of the spinal canal.In seven patients,ultrasonography was used again after circumferential decompression to compare the cross-sectional area of the spinal cord before and after circumferential decompression.Results The average period of follow-up was (12±2) months (range 9-15 months).The Japanese Orthopedic Association score was significantly higher at the final follow-up (8.5±2.1,range 3-10) than preoperatively (5.2±1.1,range 3-7; P ＜0.01).The cross-sectional area of the spinal cord was (30.8±6.6) mm2 before and (53.6±19.1) mm2 after circumferential decompression (P ＜0.01).For five patients with TSS caused by thoracic disc herniation,the levels of circumferential decompression performed corresponded to those expected preoperatively.In contrast,for eight patients with TSS caused by ossification of the posterior longitudinal ligament,on average 1.6t±0.9 fewer levels of circumferential decompression were performed than expected preoperatively.Conclusions “Cave-in” 360° circumferential decompression is an effective therapeutic option for TSS.Intraoperative ultrasonographic evaluation may reduce the levels of circumferential decompression and ensure sufficient decompression,and increase the efficacy of this surgical technique.