For a study, researchers aimed to develop and validate the Function Assessment scale for Spinal Deformity (FASD). Spinal malalignment was believed to impact daily functioning. Standard evaluation of adult spinal deformity (ASD) was based on static radiography and patient-reported scores, which failed to assess functional impairments. In order of the development of the FASD, 70 ASD patients and 20 controls were measured to identify the most discriminating items of the Balance Evaluation Systems Test and Trunk Control Measurement Scale. Discussions between experts on the clinical relevance of selected items led to further item reduction. Patients with ASD, mainly with sagittal malalignment, scored worse than controls on FASD (P<0.001) and its subscales. No significant improvement was observed 6 months postoperatively (P=0.758). FASD correlated to all patient-reported scores and SVA and PI-LL. Reliability between sessions (ICC=0.97) and raters (ICC=0.93) was excellent. Subscales also showed good to excellent reliability, except FASD 1 on “spinal mobility and balance” between sessions (ICC=0.71). FASD proved to be a valid and reliable clinical scale for evaluating functional impairments in ASD. Objective data on function and balance was found to guide physiotherapeutic cure towards improved functioning.