Low back pain is an enormous clinical and public health problem. When we use spinal instrumentation, measurements of spinal and leg alignment in the standing position are thought to be important. Knee joint pain has also become an enormous clinical problem in the increasing elderly population. However, the correlation between spinal alignment, particularly sacral inclination, and knee joint pain is not clear. We examined the correlation between lumbar lordosis, sacral inclination, and patellofemoral joint pain in elderly subjects. Three hundred and ninety-nine people aged 50-85 years were examined. Clinical findings and physical status were determined. Measurements and determination of total lordosis from L1 to S1 and sacral inclination were made from standing radiographs. The knee joints were evaluated by using the standing knee flexion angle, radiographs, and the patellofemoral (PF) joint grinding test. Thirty-nine percent of subjects were excluded because they had definite osteo-arthritis at the femorotibial joints. Fifty-eight percent of the subjects had felt low back pain within the previous 3 months, and 16% of the subjects complained of PF joint grinding pain. Sixteen percent of the subjects showed knee flexion when standing. There was a significant difference in sacral inclination between the groups with and without PF joint grinding pain ( P < 0.01). Sacral inclination was approximately 5 degrees less in the knee flexion group. A correlation between sacral inclination and PF joint pain is defined, and its prevalence in the elderly is reported. We speculate that this phenomenon is caused by changing of lumbar alignment. In addition, we think this is a new pathological concept that we call the knee-spine syndrome.