Forty-nine patients with low-back pain referred for flexion-extension radiographs due to suspicion of lumbar instability were studied to examine the relationship between the clinical presentation and the presence of radiographic instability of the lumbar spine. Patients had a mean age of 39.2 (+/-11.3) years, with a mean Oswestry score of 20.4% (+/-13.3). The median duration of symptoms was 78 days. All patients underwent both a radiographic and clinical examination. The reliability of the radiographic variables was high, while the reliability of clinical variables ranged from moderate to good. Twenty-eight patients (57%) had radiographic instability based on published norms. Clinical variables related to the presence of radiographic instability were age, lumbar flexion range of motion, total extension range of motion, the Beighton scale for general ligamentous laxity, and segmental intervertebral motion testing. The presence of at least 53 degrees of lumbar flexion or a lack of hypomobility with intervertebral motion testing resulted in a positive likelihood ratio of 4.3 (95% CI: 1.8, 10.6), for predicting radiographic instability. The results of this study indicate that various factors from the clinical examination are useful for predicting radiographic instability. If the findings of this study can be replicated, these clinical factors could be used to inform treatment decision-making without a sole reliance on radiographic assessment.