The management and follow-up of 12 patients with major aortic thrombus formation occurring in the neonatal period between 1982 and 1987 are reported. Umbilical arterial catheters were inserted in 8 of the 12 patients before thrombus formation. Two patients had congenital thrombi. Hypertension, oliguria, hematuria, and elevated blood creatinine concentration were found at the time of diagnosis of the thrombus; nine of the patients had a patent ductus arteriosus. Supportive care was instituted in seven patients who were hemodynamically stable. Five of the patients had congestive heart failure, shock, or both, and were treated with surgical thrombectomy. Thrombolytic therapy was not used in either group. The five surgically treated patients and six of seven medically treated patients survived. Ultrasound examination suggested resolution of the thrombus in all survivors in 6 to 30 days. Sequelae from thrombus formation were present in all patients at the time of discharge and included hypertension in 9 of the 11 survivors and decreased renal function in six of them. Follow-up at 1 to 3 years revealed normal blood pressure, good growth, and good renal function in 10 of the survivors.