Poikilothermia syndrome is a rare cause of intrinsic thermoregulatory failure. Patients with this syndrome regulate body temperature poorly, if at all. Recently, a patient was referred to us who had clinical evidence of poikilothermia syndrome, as well as long-standing multiple sclerosis. Computerized tomography and magnetic resonance scanning failed to identify a hypothalamic lesion. The patient was gradually warmed to sweating, and then cooled to vasoconstriction and shivering. The core-temperature thresholds triggering each defence were calculated, after compensating for the changes in skin temperature. The calculated sweating threshold was 38.3 °C (normal: 37.0 ± 0.3 °C). The vasoconstriction threshold was 34.4 °C (normal: 36.4 ± 0.3 °C). The sweating-to-vasoconstriction interthreshold range was thus ≈4 °C, which is between 10 and 20 times the normal value. The shivering threshold was 31.8 °C (normal: 35.6 ± 0.3 °C). The vasoconstriction-to-shivering range was thus ≈2.5 °C which is more than twice the normal value. The pattern of thermoregulatory failure in this patient resembled that resulting from general anaesthesia.