Victims of the sudden infant death syndrome (SIDS) have higher overall heart rates prior to death than do control infants (1). The objective of this study was to partition these heart rate differences by state and to identify any state-dependent differences in heart rate variability and respiratory rate and variability. Twenty-two recordings of electrocardiogram (ECG) and respiration from 16 infants who subsequently died of SIDS were compared with 66 recordings of age-matched control infants. Median cardiac and respiratory rate and variability were computed for each sleep state in each recording, and one-way analysis of variance tests were performed for each variable for infants less than 1 month and for infants greater than 1 month of age. Heart rate was higher in SIDS victims less than 1 month of age than in age-matched controls during all sleep-waking states. SIDS victims greater than 1 month showed higher heart rates during rapid eye movement sleep only. Heart rate variability was also diminished during waking in victims less than 1 month, but much of this difference could be attributed to increased heart rate. These results suggest that, as a group, SIDS victims differ physiologically from control infants and that these differences may be especially prominent during particular sleep-waking states.