Transaxial computed tomographic (CT) sections represent a new approach to vertebral morphometry, allowing certain measurements to be made in vivo for the first time. The cross-sectional morphology of the bodies and pedicles of L3, L4, and L5 was studied in a series of 213 vertebrae. This revealed that the pedicles of L5 arise more laterally from the body of L5 than from L3. Further, the lateral surfaces of the L5 body are inclined obliquely, unlike those of L3. L4 is transitional in form between L3 and L5, more closely resembling the former. This morphology explains the fact, hitherto unnoticed, that the lateral outlines of the pedicles and the lateral borders of the body of L5 are not normally imaged on plain anteroposterior radiographs. It is evident that pathologic changes of the lateral borders of the body of L5 may be invisible also. In cases of transitional vertebrae in the lumbosacral region the presence or absence of the lateral outlines of the pedicles and of the lateral borders of the vertebral body may be of help in identifying the vertebrae on conventional projections.