To clarify the natural course of degenerative spondylolisthesis, the mechanism and progression of disk slippage were studied clinically and radiographically in 40 patients. Progressive slippage was observed in 12 patients (30%). No progression of slippage was noted in patients who showed narrowing of the intervertebral disk, spur formation, subcartilaginous sclerosis, or ossification of ligaments. These suggest that the mechanisms of spinal restabilization prevent progression of the disease. General joint laxity was observed in many patients (65%), and this was believed to be involved in the pathogenic mechanism of this disease. There was no correlation between the clinical symptoms and progression of slippage. These findings suggest that careful consideration of the natural mechanisms of spinal restabilization as well as the natural course of the disease is important.