A healthy boy who had showed no evidence of cerebral lesions developed epileptic seizures resembling grand mal during nocturnal sleep at the age of 9 years and 2 months. Interictal EEG showed diffuse sharp waves that were maximal in the right centroparietal area and recurred at short intervals. Polysomnography for one night was performed when the patient was 9 years and 11 months old. The values of sleep variables (percentage of sleep period time for each sleep stage, sleep latency, REM density) were within normal limits. Interictal sharp waves appeared most frequently in the stages 3 and 4, and least in the stages REM and W. Partial seizure of the left face occurred with impaired consciousness in the stage 2 at around 5 a.m. On the ictal EEG, a low-voltage fast discharge was initiated in the right frontal area, followed by a diffuse rhythmic discharge of high voltage at 5-6 Hz, which was associated with a clinical seizure. The seizures completely ceased within one month after the polysomnography. Interictal sharp waves that migrated to the centrotemporal or midtemporal area disappeared at the age of 15 years and 9 months. The patient is now 19 years and 5 months of age. Although his interictal EEG and clinical seizures were atypical, we diagnose his disease as benign childhood epilepsy with centrotemporal spike based on the polysomnographic findings and the clinical and EEG follow-up.