Background: The J wave is an ECG marker of ventricular fibrillation. However, the prevalence and clinical implications of J waves in hypothermic patients remain unclear. Methods and Results: We evaluated the clinical characteristics and ECGs of patients who were admitted for accidental hypothermia (<35.0°C). J waves were defined as notches or slurs in the terminal part of the QRS complex with an amplitude ≥0.1mV. We analyzed the prevalence of J waves and the relationship between body temperature (BT) and J wave amplitude. We also examined the augmentation of J waves following variable R-R intervals in patients with atrial fibrillation. Furthermore, we assessed the incidence of ventricular arrhythmias. A total of 60 hypothermic patients were recruited (mean age, 64±9 years; 97% male). The mean BT was 31.3°C (range, 29.4–33.5°C). J waves, which disappeared after rewarming, were observed in 30 patients (50%), with a higher frequency in patients with lower BT. Higher amplitude of J waves was associated with lower BT (P<0.001). Of the 8 patients with J waves and atrial fibrillation, 4 exhibited an augmentation of J waves following a short R-R interval. Only 1 patient with J waves developed ventricular tachycardia during rewarming. Conclusions: The prevalence of J waves and their amplitude increased with the severity of hypothermia. The temporal development of J waves might not be associated with fatal arrhythmic events.  (Circ J 2014; 78: 128–134)