Dermatomally activated somatosensory evoked potentials (DSSEPs) were recorded over the scalp of 50 healthy subjects and 40 patients who later underwent exploration for herniated lumbosacral disc. DSSEPs normally consisted of three negative peaks, N40, N56, and N78, and N41, N59, and N82 after bilateral stimulation of L5 and S1 dermatomes, respectively. In the patient group, the first two peaks were often abnormal in the presence of a clinical sensory deficit, whereas the third peak tended to remain normal. At surgery, herniated disc was confirmed in all but six patients. The findings in the myelogram were misleading in eight patients, showing either false-positive or false-negative results. The DSSEPs were normal in six patients with documented disc protrusion at the level of the L5 or S1 root. The test was also normal in another case with herniation at the level of the S2 root. In this series, DSSEPs accurately predicted the level and degree of lumbosacral root involvement. In addition to myelography and other electrodiagnostic tests, the DSSEPs are valuable non-invasive diagnostic tool for the evaluation of herniated lumbosacral disc.