Spondylolysis of the lower lumbar vertebrae is a non-united childhood fracture of the arch of the vertebra, persisting into adult life. Symptoms of disabling low back pain appear in a minority of patients, usually for the first time in adulthood. This pain is considered to arise from several separate sources, one of which may be the spondylolysis ligament.The innervation of the ligament has been investigated immunohistochemically.Specimens from eight patients were divided longitudinally for histology including hematoxylin and eosin, toluidine blue, and elastic van Gieson. Histochemistry involved immunostaining for the neuropeptides: protein gene product, calcitonin gene related peptide, substance P, vasoactive intestinal peptide, and the c-flanking peptide of neuropeptide Y.Immunoreactivity to calcitonin gene-related peptide, the c-peptide of neuropeptide Y, and vasoactive intestinal peptide was identified in the ligament or in the adjacent adipose tissue.The movement that the ligament allows at the fracture site may result in stimulation of the nerve endings both in the ligament and in the surrounding soft tissue.