Segmental electrodiagnosis of compression of individual nerve roots of the cauda equina has been attempted in 45 patients, including cases of disc hernia, spondylsis and spondylolisthesis. The needle electrode was inserted into the nerve root either just lateral to the intervetebral foramen or through the posterior sacral foramen. The recordings made on stimulating a single nerve root were somatosensory evoked potential, the cauda equina action potential and the M and H waves. Mixed spinal nerve root action potentials were also recorded by stimulating the sciatic, peroneal and tibial nerves. It was shown that a diagnosis could be made from the somatosensory evoked potential, the H wave and root pain reproduction, and also the diagnosis of a subclinical compression involvement. In other words, the neurophysiological state of dysfunction of individual roots in each aspect of compression can be expressed.