The lateral transpsoas approach to the lumbar spine was developed to eliminate the need for an anterior-approach surgeon and retraction of the great vessels and has the potential for shorter operative times. However, the reported complications associated with this approach vary. We identified the incidence of complications associated with the lateral transpsoas approach to the lumbar spine. We retrospectively reviewed 45 patients who underwent a lateral transpsoas approach to the spine for various diagnoses between January 1, 2006, and October 31, 2010. The patients' average age was 63.3 years. Sixteen (35.6%) patients had prior lumbar spinal surgery. Twenty-one patients (46.7%) underwent supplemental posterior instrumentation. Minimum followup was 0 months (mean, 11 months; range, 0-34 months). Eighteen of the 45 patients (40%) had complications: 10 (22.2%) developed postoperative iliopsoas weakness, three had quadriceps weakness, and one experienced foot drop. Eight patients (17.8%) developed anterior thigh hypoesthesia, which did not fully resolve in seven of the eight patients at an average of 9 months' followup. Three patients had postoperative radiculopathies, one a durotomy, and one died postoperatively from a pulmonary embolism. We found a 40% incidence of complications and a nontrivial frequency and severity of postoperative weakness, numbness, and radicular pain in patients who underwent a lateral transpsoas approach to the spine. Given the expanding use of the approach, a thorough understanding of the risks associated with it is essential for patient education, medical decision making, and identifying methods of reducing such complications. Level IV, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.