BACKGROUND J waves are abnormal electrocardiogram findings that indicate an elevation at the junction between the QRS complex and the ST segment. Hypothermia is associated with fetal arrhythmia, along with the increase of J-wave manifestation. METHODS A 68-year-old woman with a medical history of old cerebral infarction and dementia was admitted to the emergency department with accidental hypothermia. An admission, electrocardiogram (ECG) showed prominent J waves with the highest amplitude recorded in limb and precordial leads. Continuous ECG monitoring was recorded during the rewarming therapy. As the body temperature increased, the amplitude of J waves became lower and disappeared. This case clearly showed that the degree of core body temperature is related to the height of the J-wave amplitude. There was no fatal arrhythmia during rewarming therapy. CONCLUSIONS This case describes serial changes in the J-wave amplitude and morphology during rewarming therapy. Continuous ECG monitoring is important in a patient with severe hypothermia.