In this study dermatomal somatosensory evoked potentials (DSEPs) were investigated for their contribution to the diagnosis of lumbosacral disc herniation. Forty-three patients with a diagnosis of lumbar disc herniation at the L4-5 or L5-S1 levels, which is demonstrated by CT, and 49 healthy controls were studied prospectively. L5 and S1 DSEPs, and peroneal somatosensory evoked potentials were recorded in both groups. We determined 3 criteria for DSEP abnormality compared to control data: 1) No response in L5 or S1 DSEPs, 2) P1 peak latency longer than 2,5 standard error on the height-latency regression line, 3) Interlimb latency difference more than 2,5 msec for either L5 or S1 DSEPs or L5 and S1 latency difference more than 2,9 msec on the same limb. We found DSEP abnormalities in 27 of 43 patients (63%). Out of 27 abnormal DSEPs were accordant with localization of disc herniation in 24 (89%), with lateralization of disc herniation in 20 (74%), and with localization and lateralization in 18 (67%). We concluded that DSEPs, as a noninvasive method, can contribute to determine the localization of pathology in lumbar disc hernias.