OBJECTIVE--To determine whether there was a difference in the relation between heart rate and RT intervals in infants who later died of sudden infant death syndrome (SIDS) and controls. DESIGN--A blinded, computer analysis of prospectively acquired physiological data on SIDS cases and controls. SETTING--Physiological data obtained from infants at home (collaborative analysis National Heart and Lung Institute and Exeter University). PARTICIPANTS--Nine fullterm infants who subsequently died of SIDS and 10 surviving controls matched for age and birth weight. INTERVENTIONS--24 hour tape recordings of the electrocardiogram at home between three and 11 weeks of age. MAIN OUTCOME MEASURES--Mean value of the constant b, which relates by linear regression the log of each RT interval to the log of its proceeding RR interval, and any difference between SIDS cases and controls. RESULTS--The mean (2SEM) values for b were 0.20 (0.03) for the 10 controls and 0.19 (0.03) for the nine SIDS infants. Cyclical correlations between RT and RR intervals of varying strength were identified in both SIDS infants and controls. CONCLUSIONS--Infants who subsequently died of SIDS did not show an impaired ability to modify RT intervals in response to change in heart rate.