Summary An impaired ability to arouse from sleep may play an important role in the pathogenesis of sudden infant death syndrome (SIDS). This study aimed to investigate the effects of prone sleeping on the nature of both induced and spontaneous arousal responses in infants. Thirteen healthy term infants were studied longitudinally at 2–4 weeks, 2–3 months and 5–6 months postnatal age. A pulsatile jet of air to the nostrils was used to induce arousal from both active sleep and quiet sleep in both prone and supine positions. For each stimulus, arousals were classified as sub-cortical activations and cortical arousals, scored using physiological and electroencephalogram changes and expressed as a percentage of the total number of arousals. Spontaneous arousals were similarly analysed. Increased proportions of cortical arousals, hence decreased proportions of sub-cortical activations, were observed in the prone position at 2–3 months. This distinct peak in the proportion of cortical arousals occurred regardless of sleep state and regardless of whether the arousal occurred spontaneously or was induced by air-jet stimulation. The nature of arousal responses in healthy term infants is altered in the prone sleeping position at 2–3 months after birth, the age where SIDS incidence is highest. We postulate that a greater propensity for cortical arousal may be a protective mechanism to promote complete arousal in a vulnerable sleeping position and/or a vulnerable period of maturation. Inadequate or incomplete cortical arousals may explain the increased risk of SIDS associated with the prone position at this age.