Although the etiology of most degenerative changes in the lumbar spine is unclear, genetic factors may play an important role. To investigate this link, we reviewed magnetic resonance images of the lumbar spines of identical twins to assess the degree of similarities in degenerative findings in the discs. Observers who were blinded to twinship evaluated sagittal T1-weighted and T2-weighted magnetic resonance images of the lumbar spines of forty male identical twins (twenty pairs) with respect to changes in the end plates, desiccation of the discs, bulging or herniated discs, and decrease in the height of the disc space. Similarities between co-twins were significantly greater than would be expected by chance. Whereas smoking status and age explained 0 to 15 per cent of the variability in the various degenerative findings in the discs, 26 to 72 per cent of the variability was explained with the addition of a variable representing co-twin status. These findings are compatible with a marked genetic influence and warrant further investigation.