The lateral transpsoas approach to access the vertebrae obviates the need for an approach surgeon and minimizes muscular disruption, thus allowing for quicker recovery. Several reports on the lateral transpsoas procedure have described few complications. However, the development of an unsightly and painful abdominal flank bulge is a largely under-recognized and very rare complication of the lateral transpsoas approach. A 59-year-old man suffered from back pain and bilateral posterior leg pain. Computed tomography (CT) scan and MRI showed retrolisthesis at L3-4, L2 wedge vertebrae with kyphosis, left L4 screw loosening, and L3-4 disc herniation with central canal stenosis. L2 corpectomy and L3-4 DLIF and posterior fusion to T12 for kyphosis correction were performed. For the lateral approach, resection of the T11 rib was performed. One month later, he developed left abdominal flank bulging below the lateral approach site, which was aggravated by walking, coughing, defecating, constipation, and eating. CT scan showed left abdominal flank bulging accompanied by abdominal muscle thinning. We believe that this complication is caused by denervation of the abdominal musculature after injury to the T11 intercostal nerves.