The natural history of radiographic instability of the lumbar spine was determined by reexamination of 50 patients who had been diagnosed with this condition at least 10 years previously and correlation of these findings with changes in the clinical condition. The initial diagnosis and follow-up examination was performed by dynamic radiography, and the following observations were made: 1) radiographic instability resolved spontaneously in 20% of cases; 2) radiographic instability due to isolated posterior opening (asymmetric anterior collapse of the intervertebral disc in flexion) usually was self-limited; and 3) combined posterior opening and forward translation in flexion was associated with chronic instability and debilitating symptoms. The functional prognosis, whether or not symptoms are improved, in patients with radiographic lumbar instability depends primarily on developmental narrowing of the spinal canal, rather than on the existence or type of radiographic instability.