The electrocardiographic changes observed in (short-term) induced and accidental hypothermia are well known. We studied the electrocardiographic effects of steady mild hypothermia in 4 female patients with poikilothermia. 24h Holter recording revealed during mild hypothermia significantly decreased heart rate and prolongation of the QT interval in comparison with normothermia. During hypothermia the short-term heart rate variability and the circadian rhythm of heart rate variability were significantly higher than during normothermia. We conclude that in our patients even mild steady hypothermia induces electrocardiographic alterations. In patients with disorders of thermoregulation one should be on the alert for cardiovascular complications, especially in cold climates.