The case histories of two infants with aortic thrombosis are presented. Clinically, an obstructive aortic arch lesion was suspected because of a systolic pressure gradient between the upper and lower extremities in one infant and unequal peripheral pulses with lower limb cyanosis in the other. Noninvasive studies were inconclusive, the only abnormality noted echocardiographically being severe left ventricular dysfunction. Cardiac catheterization including angiography defined the severity of the obstruction and location of the thrombus in both babies. The thrombus originated within a ductus arteriosus aneurysm in one infant and was located in the ascending aorta and transverse arch in the other. Although successful surgical removal was accomplished, both babies died unexpectedly in the early postoperative period.