Patients with parenchymatous mass lesions in the perirolandic area as seen by CT or MRI are unique in that identification of the central sulcus (CS) intraoperatively is crucial in determining the surgical strategy. Somatosensory-evoked potential (SEP) responses were used under general anaesthesia for the intraoperative identification of the central sulcus (CS). Sixty-five patients with parenchymatous mass lesions were included in the study and a phase reversal in the hand area across the central sulcus was used to identify this sulcus. Based on the clinical findings the patients were classified into three groups: Group A were patients with predominant motor deficits, Group B, patients with predominant sensory deficits; and Group C were patients with gross sensory and motor deficits. This study shows that in patients with predominant motor deficits a phase reversal with good amplitude was obtained in all cases. In patients with predominant sensory deficits a phase reversal could be obtained, but their amplitudes were markedly decreased. In patients with predominant sensorimotor deficits no phase reversal could be obtained. The central sulcus could not be located accurately by somatosensory evoked potentials in these cases (group C).