Victims of sudden infant death syndrome (SIDS) have occasionally been reported to have had prolonged apnea or an increased frequency of short apnea prior to their deaths. To examine the extent of these abnormalities, we compared pneumograms obtained in 17 infants who subsequently died of SIDS (10 with history of apnea) with those of 34 age- and sex-matched controls. The recordings were analyzed by a computer program that avoided observer bias. SIDS infants had significantly greater mean heart rate (P less than 0.05) and periodic breathing during quiet time (P less than 0.003) than control infants had. The apnea in SIDS infants tended to be more periodic than in control infants (P less than 0.002). In addition, the incidence of bradycardia was greater in SIDS (three infants) than in controls (none) (P less than 0.03). These differences suggest a disturbance of autonomic function prior to death in some victims of SIDS.