Ninety-eight consecutive patients with clinically suspected congenital heart disease were prospectively studied with electroencephalographic (EEG) recordings before cardiac catheterization. Twenty-five patients had abnormal EEGs. Fifty-five patients had acyanotic heart disease and normal neurologic examination results, of whom 15 had abnormal EEGs. Thirteen had spikes or spike and wave discharges and two had mildly abnormal EEGs. Twenty-seven patients had cyanotic heart disease and normal neurologic examination results, of whom four had abnormal EEGs. Seven patients (8%) had abnormal neurologic examination results, of whom four had abnormal EEGs. There was a higher incidence of seizures with increasing age. Based on their medical history two children with acyanotic heart disease had had seizures without EEG abnormalities. Five children with normal catheterization findings and four children with Down's syndrome (two with abnormal EEGs) were excluded from the total of 98. These findings suggest that subclinical nervous system involvement may occur in congenital heart disease.