The quantities of sinus tachycardia in 24-h recordings of the electrocardiogram from 16 full-term infants (greater than or equal to 37 weeks gestation) who were subsequently victims of the sudden infant death syndrome (SIDS), from 230 randomly selected age-matched full-term survivors and from 64 full-term survivors matched for age and birth weight were measured by computer and manual analysis techniques. Of 16 infants dying of SIDS, 7 had elevated levels of sinus tachycardia (greater than 95th centile in controls) (P less than 0.01). Although high levels of sinus tachycardia might be of value in identifying infants at high risk of SIDS, these encouraging findings must first be validated by further prospective studies.