this longitudinal observational study investigated the relationship between changes in spinal sagittal alignment and changes in lower extremity coronal alignment. A total of 58 female volunteers who visited our institution at least twice during the 1992 to 1997 and 2015 to 2019 periods were investigated. We reviewed whole-spine radiographs and lower extremity radiographs and measured standard spinal sagittal parameters including pelvic incidence [pi], lumbar lordosis [LL], pelvic tilt [pt], sacral slope [SS] and sagittal vertical axis [SVA], and coronal lower extremity parameters including femorotibial angle (ftA), hip-knee-ankle angle (HKA), mechanical lateral distal femoral angle (mLDfA), mechanical medial proximal tibial angle (mMptA) and mechanical lateral distal tibial angle (mLDtA). Lumbar spondylosis and knee osteoarthritis were assessed using the Kellgren-Lawrence (KL) grading system at baseline and at final follow-up. We investigated the correlation between changes in spinal sagittal alignment and lower extremity alignment and changes in lumbar spondylosis. The mean age [standard deviation (SD)] was 48.3 (6.3) years at first visit and 70.2 (6.3) years at final follow-up. There was a correlation between changes in PILL and FTA (R = 0.449, P < 0.001) and between PILL and HKA (R = 0.412, P = 0.001). There was a correlation between changes in lumbar spondylosis at L3/4 (R = 0.383, P = 0.004) and L4/5 (R = 0.333, P = 0.012) and the knee joints. Changes in lumbar spondylosis at L3/4 and L4/5 were related to changes in KOA. Successful management of ASD must include evaluation of the state of lower extremity alignment, not only in the sagittal phase, but also the coronal phase. In recent years, with the aging population, the number of patients presenting to consultation rooms with both knee osteoarthritis (KOA) and lumbar degenerative diseases has increased 1. In adult patients with spinal deformity, spinal kyphosis changes are caused by disc degeneration and vertebral fracture. A previous study reported that spinal deformity in the sagittal phase is progressive, and influences pain, risk of falls, and health-related quality of life (HRQOL) 2-5. Barry et al. reported the compensatory mechanisms associated with adult spinal deformity, and reported that in advanced spinal deformity, the body is affected from head to toe, including changes in hip extension and knee flexion in the sagittal phase to maintain spinal alignment 6. On the other hand, KOA, which causes knee pain, knee contracture, decreased muscle strength, and decreased QOL, induced knee flexion in the sagittal phase and varus deformity in the coronal phase in the lower extremities, which is one of the characteristic changes proceeding KOA 7-9. Both the sagittal phase and the coronal phase are very important in the evaluation of KOA. Knee-spine syndrome has been previously reported by Murata et al. and Itoi who indicated that deformity of the knee joint influences spine deformity 10,11. However, their studies did not examine the coronal phase of