Three patients who had suffered episodes of spontaneous hypothermia are described. Circulatory control was normal for their age but there was evidence of a persistent defect of thermoregulatory mechanisms. In all patients vascular thermoregulatory reflexes dependent upon peripheral receptors were active and in two patients reflex shivering could be obtained. There appeared, however, to be a failure of central thermoregulation as the vascular reflexes were active at abnormally low central temperatures and a fall in central temperature did not initiate shivering. It is concluded that in patients who have suffered spontaneous hypothermia there may be a persistent defect of central mechanisms subserving vasomotion and shivering, although thermoregulatory reflexes dependent upon peripheral receptors can be active. The reflex pathways for thermoregulation in man therefore have a separate integrity, physiologically independent of the central thermoregulatory mechanisms.