Sympatheticovagal imbalance causing episodes of severe bradycardia has been suggested as a cause of apparent life threatening events (ALTEs). The autonomic control of the heart rate in 18 infants with ALTEs and 12 controls was evaluated by the head upright tilt test. Five different heart rate response profiles (compared with the baseline) were observed during the tilt: (1) increase followed by a decrease and return to baseline; (2) sustained increase; (3) decrease followed by an increase and return to baseline; (4) sustained decrease; (5) no change. Eighty eight per cent of controls responded with heart rate increase followed by decrease or sustained increase compared with 55% of infants with an ALTE; a significantly greater proportion of infants with ALTEs than controls responded with heart rate decrease or no change in rate (45% v 8%). This altered reaction during a head upright tilt test may be an expression of an underlying autonomic dysfunction in infants who have experienced an ALTE.