A clinical and radiological follow-up study with at least 20 years of observation was made of 255 spondylolysis and spondylolisthesis patients for examination of the clinical course and its possible correlation to radiographic findings. The progression of slipping was small and not correlated to age at diagnosis and initial degree of spondylolisthesis. Disc height reduction at the spondylolytic level occurred at an earlier age and was more severe than in a normal control group. Symptoms were correlated to radiographic pathology. Risk factors for low-back symptoms were greater than 25% slipping, low lumbar index in L5 spondylolysis, spondylolysis at the L4 level, and early disc degeneration.