Although tonic spasms (TS) observed in West syndrome(WS) are regarded as generalized seizures, the mechanism of their occurrence has not yet been fully elucidated. We analyzed ictal EEGs of TS to determine the mechanism and to clarify the role of cortical pathology in the occurrence of TS. Of 53 infants with WS, we classified cryptogenic cases as Group Ⅰ. Symptomatic cases were classified into 2 groups : Group Ⅱ consisted of those without partial seizures during the period of WS and Group 3 consisted of those with partial seizures. We analyzed 7283 ictal EEGs of TS regarding the following 5 points : 1) patterns of ictal EEGs, 2) frequency of isolated TS, 3) asymmetry of ictal EEGs, 4) spasm-spasm intervals, and 5) duration of ictal EEGs of each TS. We concluded that the asymmetry of ictal EEGs and the "rhythmic slow activity" immediately following ictal EEGs of TS were related to cortical pathology. Since "rhythmic slow activities" were often observed in Group Ⅲ and appeared at the same regions as those in ictal EEGs of concomittant partial seizures, they were considered to be peculiar EEG patterns related to cortical pathologic lesions.