Somatosensory evoked potentials (SEPs) were recorded from the scalp in 54 normal subjects following stimulation of the posterior tibial nerve and the skin of the L5 and S1 dermatomes. Criteria of abnormality were defined and used to test the accuracy of the technique in 21 surgically proven cases of lumbosacral root compression. A sensitivity of 95% was achieved with dermatomal SEP, while the posterior tibial nerve was of limited value. In 100 patients with problematic sciatica, dermatomal SEPs were an important adjunct to conventional diagnostic procedures not only diagnostically but also in providing a prognostic index of the likely outcome of therapy. Application of the technique promises to reduce the incidence of failed back syndrome. Dermatomal SEP was useful in confirming the diagnosis of radiculopathy especially in patients with equivocal radiology. Operative recording of sensory nerve root action potentials to dermatomal and posterior tibial nerve stimulation was performed in 13 patients, to study the degree of overlap of the dermatomes. The results suggested minimal overlap at the signature areas of lumbosacral dermatomes. Recordings from compressed nerve roots across a prolapsed disc in 4 patients disclosed conduction abnormality akin to that observed in acute or chronic compression neuropathy.