Fifty patients with back pain and radiologically diagnosed spondylolysis were evaluated by a single-photon emission computed tomography (SPECT bone scanning). These patients were separated into three groups according to the degree of spondylolisthesis accompanying the spondylolysis. The data obtained from the study indicate that in acute spondylolysis, the SPECT scan is positive at the pars interarticularis. As the spondylolysis becomes chronic, the SPECT scan tends to revert toward normal even though healing of the spondylolysis has not occurred. As spondylolisthesis develops and progresses, the SPECT scan again becomes positive. The positivity, however, is more anterior and more diffuse. The authors propose that SPECT scanning in spondylolysis is not a positive or negative process, but rather varies with the time and stability of the spondylolytic spine.