The concurrence of early disc degeneration and abnormal segmental motion in spondylolysis of young low back pain patients (n = 14) was investigated using magnetic resonance imaging (MRI) and flexion-extension radiography. Seven patients with L5 spondylolisthesis had normal discs on MRI and 7 had disc degeneration below the slipped vertebra. The parallel motion, angular mobility and centre of motion were similar in the degenerated discs and in normal discs. The present study shows that in adolescents the motion pattern and hydration of discs associated with spondylolysis and spondylolisthesis are not always abnormal. The early dehydration and degeneration of lumbar discs observed on MRI are not always associated with abnormal mobility of the corresponding motion segment.