Power spectral density measured from central cortical EEG recordings during sleep was compared in two groups of 20 infants each at intervals between 1 week and 6 months of age. One group consisted of siblings of infants who had previously died of the sudden infant death syndrome, as determined by history and autopsy report (sibling group). The second group was made up of age, sex, and socioeconomically matched infants with no familial history of sudden infant death (control group).