Inconsistent data exist regarding the anatomy of the spinal nerve roots lateral to the thecal sac. A newly developed in situ technique was used to precisely define anatomic parameters on 20 fresh human cadavers. The take-off angle of the nerve roots from the thecal sac decreases from a mean of approximately 40 degrees from L1-L5 to 22 degrees at S1. The motor bundles are directly ventral to the sensory fibers within individual roots extrathecally. Dorsal root ganglia size varies with vertebral level. The majority of ganglia lie directly beneath the vertebral pedicles and one third overlie a portion of the lateral intervertebral disc. These previously undescribed relationships may aid in the understanding of lumbosacral neurocompressive disorders and are important to note during pedicle screw insertion, posterolateral decompression for spinal trauma, and paravertebral approaches for lateral disc herniations.