Bertolotti's syndrome refers to the association of back pain with lumbosacral transitional vertebrae. Such vertebrae were observed in 140 of 2,000 adults with back pain over a 4-year period of study. Each patient had radiographic evaluation of the lumbar spine by plain films as well as a sectional imaging modality (magnetic resonance [MR] or computed tomography [CT]). The overall incidence of structural pathology (eg, spinal stenosis and disc protrusion) detected by CT or MR was not apparently higher in patients with transitional vertebrae, but the distribution of these lesions was significantly different. Disc bulge or herniation, when it occurred, was nearly nine times more common at the interspace immediately above the transitional vertebra than at any other level. Spinal stenosis and nerve root canal stenosis were more common at or near the interspace above the transitional vertebra than at any other level. Degenerative change at the articulation between the transverse process of the transitional vertebra and the pelvis was an uncommon occurrence; when seen there was no significant correlation with the reported side of pain. It is postulated that hypermobility and altered stresses become concentrated in the spine at the level immediately above a lumbar transitional vertebra. Accelerated disc and facet joint degeneration at this level may then result.