Five vertebrae with unilateral spondylolysis are presented. The associated asymmetry of the posterior elements supports the concept of a localised form of growth deficiency. The defect is difficult to demonstrate radiologically, and is perhaps present more frequently than is recognised. It should be suspected clinically from asymmetry of the neural arch and from unilateral wedging of the vertebral body, and may be demonstrated by further radiographic examination. The clinical significance is uncertain, but one patient is presented in whom unilateral spondylolysis was associated with intermittent sciatic pain.