Electroencephalograms (EEGs) were recorded on 36 infants who were treated with arteriovenous extracorporeal membrane oxygenation (ECMO) between 1986 and 1989. Twelve of 36 infants had EEGs prior to and during ECMO. Twenty-one infants who met ECMO criteria but were treated with mechanical ventilation only served as a comparison (COMP) group. Electrographic seizures occurred in 7 of the 36 (19%) ECMO patients and in 3 of the 21 (14%) COMP patients. Five of 7 ECMO infants had electrographic status epilepticus. Three infants developed electrographic seizures during ECMO in association with an acute cerebral injury. These patients did not have significant hemispheric predominance in the origin of electrographic seizures (2 right, 3 left and 2 bilateral). Repetitive or periodic discharges (RPD) with frequencies between 0.5 and 5 Hz were seen after starting therapy in 23 of the 36 (64%) ECMO patients and 15 of the 21 (71%) of the COMP group. Only in the ECMO patients did RPD arise significantly more frequently from the right hemisphere (13 right, 4 left and 6 bilateral; p = 0.015). Neuroimaging studies showed evidence of ischemic lesion in 8 of the 36 (22%) ECMO patients with significantly right-sided predominance (5 right, 1 left and 2 bilateral; p = .05). Cerebellar hemorrhages occurred in 3 ECMO patients. Ischemic lesions occurred in 3 of the 21 (14%) COMP patients, all occurring on the left side. In the ECMO group, severe outcome or death was significantly associated with the presence of electrographic seizures, status epilepticus, and suppression-burst pattern (p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)