The Osborn (J) wave refers to a dome-shaped deflection on the electrocardiogram, that appears as a late delta wave following the QRS complex or as a small secondary R wave (R'). A 25-year-old man was found unconscious in the open in cold and snowy weather. Electrocardiogram obtained on admission showed atrial fibrillation with a normal ventricular rate, increased duration of the QRS complex and the QT interval, and the J wave in all derivations. Rectal temperature was measured as 27.1°C. In addition, cranial tomography showed subdural hematoma in the left parietal region. Treatment for subdural hematoma and hypothermia resulted in normal sinus rhythm and disappearance of the J wave on the electrocardiogram obtained in the 24th hour. However, he developed brain death due to deterioration in the intracerebral pathology at 30 hours.