Electrocardiograms obtained during sleep within the first and/or fourth week postnatally were available on eight infants who subsequently died of the sudden infant death syndrome (SIDS). The corrected QT (QTc) and R-R intervals were compared with controls for the purpose of evaluating their relevance for SIDS. The QTc interval in controls increased with age and tended to be longer during sleep without rapid eye movements. The R-R interval decreased with age. None of the SIDS victims was found to have a prolonged QTc interval. However, the R-R interval during rapid eye movement epochs was significantly shorter in future SIDS victims compared with controls. These data were interpreted as being inconsistent with the congenital variants of the long-QT hypothesis but compatible with the growing conviction that infants who die of SIDS have a chronic underlying abnormality that has subtle manifestations within the immediate postnatal period.