To evaluate neurological involvements in hypomelanosis of Ito (HI), we conducted electroencephalographic and evoked potential studies in 3 children with HI. Patient 1 presented with the West syndrome, and was severely handicapped. Patient 2 had mild mental retardation. Patient 3 appeared neurologically normal, but chromosome analysis showed a 46,XX/46,XX,12p + mosaic karyotype. EEGs of Patient 1 initially showed asymmetric hypsarrhythmia, and later diffusely increased fast waves, occipital dominant alpha rhythm plus multifocal spikes. Patients 2 and 3 had minimal and no EEG abnormalities, respectively. Brainstem auditory evoked potentials showed prolongation of wave I latency as well as I-III and I-V interpeak latencies in Patient 1, and prolonged wave I latency in Patient 3. Flash visual evoked potential studies disclosed a significant difference between the right and left latencies of wave IV in Patient 2. The fact that there were no specific EEG or evoked potential findings for HI seems to be consistent with the pathogenetically heterogeneous nature of this disorders. We emphasize the usefulness of neurophysiologic examinations in evaluating diverse CNS dysfunctions in HI patients.