Hypercarbic and hypoxic arousal responses were measured in 22 normal and 11 near-miss sudden infant death syndrome infants during natural sleep. End-tidal PCO2, (PACO2), transcutaneous PO2, electrocardiogram and heart rate, and thoracic and abdominal circumference were continuously recorded. The essential behavioral criteria for arousal were eye opening and crying. For each hypercarbic arousal test, step increases in FICO2 were made until arousal occurred or until PACO2 reached 65 mm Hg. For each hypoxic arousal, step decreases in FIO2 were made until arousal occurred or until FIO2 = 0.15 had been maintained for 20 minutes. The hypercarbic arousal threshold was significantly higher in near-miss SIDS than in normal infants, 54.9 +/- 2.3 vs 48.4 +/- 1.4, respectively ((P less than 0.05). An arousal response to hypoxia occurred in only 9% of near-miss SIDS infants compared to 70% of normal infants (P less than 0.01). The level of respiratory chemostimulation required to produce an arousal response from sleep is significantly greater in near-miss SIDS than in normal infants. We speculate that deficient arousal responsiveness, especially to hypoxia, may prevent potential SIDS victims from responding appropriately to apneic asphyxia.