We have observed an electrocardiographic (ECG) pattern of deep T-wave or "giant T wave" inversion in patients with acute coronary insufficiency. We reviewed the ECGs of 936 patients admitted to our coronary care unit during a one-year period. We found the pattern of giant T-wave inversion was present in nine patients (1 percent). We examined the echocardiograms of all of these patients and we analyzed the coronary angiograms on the seven patients in whom it was performed. We found that giant T-wave inversion was usually found in patients with stenosis in the left coronary system. In addition, the majority of these patients also had echocardiographic evidence of left ventricular hypertrophy. We conclude that the high frequency of a partially patent vessel in the left coronary system suggests that this ECG pattern may be useful in identifying patients who might benefit from coronary revascularization.