Hypercarbic and hypoxic arousal responses during sleep were measured in healthy term infants, infants where a previous sibling died from sudden infant death syndrome (SIDS) and infants suffering a clearly defined apparent life threatening event (ALTE) requiring vigorous or mouth to mouth resuscitation. Groups of infants were tested at approximately one, six and 13 weeks postnatally. Arousal was defined as gross body movement with eyes opening and moving or crying. Hypercarbic arousal was by step increases in F1 Co2 until arousal occurred or until endtidal (PETCO2) reached 8.7 KpA (65 mm Hg) Hypoxic arousal was by step decreases in FIO2 until arousal occurred or until an FIO2 of 0.15 had been maintained for 20 minutes. There was no difference in hypercaribic arousal threshold with age in any group. Hypercarbic arousal threshold was significantly higher in siblings (mean 53.4, 53.6, 54.7 mmHg. [7.12, 7.14, 7.29 KPA] at 0, 6, 13 postnatal weeks) compared to controls (mean 50.9, 52.3, 53.0mm Hg. [6.78, 6.97, 7.29 KPS respectively). ALTE infants differed only at 12 weeks having a significantly lower threshold (51.0mmHg. [6.80 KPA] V 53.0mm Hg. (7.06 KPA]) compared to controls. There was no difference in hypoxic arousal response with age in any group. An arousal response to hypoxia occurred in only 22% of ALTE infants and 40% of siblings compared to 67% of normal infants. Deficient sleep arousal, especially to hypoxia, is common in infants and especially those considered at increased risk from SIDS. This deficiency is present in the first postnatal week and did not vary overy the first three months of postnatal life.