To elucidate the pathophysiology of spasms in series, the distribution and density of spikes on hypsarrhythmia were studied in 13 patients with West syndrome by excluding slow waves from EEG using a digital filter. (1) Interictal spikes were mostly multifocal and dominant over the bilateral posterior head area with very few diffuse discharges. Therefore, spikes on hypsarrhythmia were demonstrated to occur actually depending on the brain regions although they appeared random. (2) The dominant region of interictal spikes did not correspond to the underlying focal cortical lesion in many patients, and was indicated to reflect the general process of cerebral maturation in infancy. (3) There were far fewer inter-spasm spikes, especially during the middle phase of a series of spasms, than interictal spikes. It was implied that the cortical activity of hypsarrhythmia was interfered with by the abnormal subcortical function, which might be related to the generation of spasms. (4) The dominant region of inter-spasm spikes tended to coincide with a focal cortical lesion shown by MRI and the focus of associated partial seizures. (C) 2003 Elsevier B.V. All rights reserved.