Five patients with clinically definite multiple sclerosis are reported who presented with acute relapses associated with hypothermia. Repeated episodes of hypothermia were seen in four. Thrombocytopenia was associated with the hypothermia in four patients. Further investigation disclosed a tendency to chronic hypothermia and suggested an altered thermoregulatory set point in one patient, when MRI, endocrine, and autonomic studies failed to localise a lesion in the hypothalamus, but subsequent necropsy showed hypothalamic lesions. In such patients a predisposition to altered thermoregulation may occur due to direct involvement of the hypothalamus or from combined lesions affecting hypothalamic outflow to the brainstem and spinal cord.