Electrocardiograms of 50 patients with accidental hypothermia were reviewed with regard to the J wave with the following results: (1) J waves were observed in 40 of 50 cases. (2) J waves were recorded most frequently in leads II or V6 (34 of 40 cases, 85%). However, in deep hypothermia, the J wave was often most prominent in leads V3 or V4. (3) The size of the J wave appeared to be related to body temperature. Below 30 degrees C, large J waves were often observed; above 30 degrees C, J waves were usually smaller. (4) J waves were not distinctive in the cases with clockwise rotation. (5) The appearance and the size of the J waves seemed not to be associated with the arterial pH. (6) The J wave decreased in size along with rise of the body temperature. However, a small J wave persisted in many cases even after normothermia was restored. It was also difficult to distinguish these small J waves from small notches at the QRS-ST junction which are sometimes observed in normal individuals not subjected to hypothermia.