STUDY DESIGN: Retrospective measurement of spinal and pelvic parameters in adult spinal deformity patients. OBJECTIVE: To correlate spinal and pelvic parameters in adult spinal deformity patients who were in neutral spinal balance. SUMMARY AND BACKGROUND DATA: It is believed that sagittal spinal balance is influenced by both spinal and pelvic parameters, which are closely interrelated as manifested by the reciprocal changes seen when any of the interrelationships was altered. New parameters including proximal thoracic slope (PTS), proximal thoracic tilt, thoracic apical tilt, and coxo-spinal angle (CSA) were studied and correlated with previously studied spinal and pelvic parameters. METHODS: One thousand patients who had undergone standing scoliosis views from 2007 to 2010 were screened. A total of 70 patients, 29 with a diagnosis of degenerative scoliosis and 41 with the diagnosis of adult idiopathic scoliosis, were analyzed for various spinal and spinopelvic parameters. Linear regression analysis was performed. RESULTS: Thoracic kyphosis (TK) plus sacral slope (SS) had as strong a correlation with lumbar lordosis (LL) (r = 0.871; p < .000) as with pelvic incidence. The ratio LL / (TK + SS) yielded a constant ratio of 0.74 for the balanced spine. Pelvic incidence ± 9 = LL. Lumbar lordosis × 0.74 = TK. Coxo-spinal angle correlated with TK (r = 0.404; p = .000) and CSA / TK yielded a constant ratio in balanced spines. Proximal thoracic slope and thoracic apical tilt strongly correlated with TK (R = 0.793; p = 0.000). Proximal thoracic slope allows introduction of the spinal equation, PTS + LL = SS + TK, which is based on the geometric principle that when measuring angles between 2 horizontal parallel lines the sum of the angles in opposite directions is equal. CONCLUSION: The spinal equation may predict ideal spinal and pelvic parameters that may aid in preventing complications such as proximal junctional failure.