In 66 patients with back pain and suspected spondylolysis, the results of bone scintigraphy have been correlated with operative findings and clinical follow-up. Although bone scintigraphy is of little value for primary diagnosis, it helps to distinguish between those patients with established non-union of the defect, and those in whom healing is still progressing and who may benefit from immobilisation. We also found that increased uptake on the contralateral side to a unilateral spondylolysis is suggestive of impending fracture.