The physical signs of lumbar instability have not yet been clearly defined. Furthermore, they do not always coincide with the abnormal movement on flexion—extension radiographs. Thus, the treadmill was used to evaluate clinical lumber Instability. The subjects were 82 patients with degenerative lumbar disease aggravated by walking or daily physical activities. The reproduction and provocation of low-back pain or pain in the lower extremities was examined during walking on a treadmill. The responses of the reproduction and provocation of the symptoms were divided into four grades. Fifteen cases were examined again using a treadmill after body casting to determine the indication of fusion surgery. As a result, the responses in the reproduction or provocation test using a treadmill manifest more dominant reflection of clinical symptoms suggesting clinical lumbar instability than the existence of the abnormal movement on flexion-extension radiographs. Reexamination after body casting was useful to determine the indications of fusion operations. The reproduction or provocation test using a treadmill was implied as a possible functional evaluation method of clinical lumbar instability.