To clarify the mechanism of clustered spasms in West syndrome (WS), we examined ictal SPECT and EEG, as well as those during the interictal period, in three patients with symptomatic WS who had apparent focal cerebral lesions. For ictal SPECT and EEG, we monitored the patients with EEG, and as soon as we noticed the occurrence of clustered spasms clinically and electroencephalographically, [(99m)Tc]ECD was injected intravenously and flushed thoroughly with saline. In these three patients, regional cerebral blood flow (rCBF) increased during ictus and decreased during the interictal period in the area that coincided with the focal cerebral lesion recognized by CT/MRI. The ictal hyperperfusion of bilateral basal ganglia was also detected in two of the three patients. The ictal EEG showed a diffuse slow wave complex corresponding to a clinical spasm. The sharp waves that preceded the diffuse slow wave complex and the spasm appeared in the same area in which rCBF increased during ictus. None of the patients showed partial seizure before or after clustered spasms clinically or electroencephalographically during the ictal SPECT study. Secondary generalization from a cerebral focus may explain the mechanism of spasms in these patients with WS: focal cortical discharge may primarily generate clustered spasms and trigger the brainstem and basal ganglia structures to produce spasms.