Eighty-one neonates were prospectively studied by sonography of the abdomen to determine the frequency, risk factors, clinical symptoms, complications, and natural history of aortic thrombosis seen in conjunction with the use of umbilical-artery catheters. Twenty-six percent of neonates developed sonographically detectable thrombosis. Twenty-nine percent of the neonates with thrombosis were asymptomatic, and an additional 24% had hematuria (diagnosed by dipstick) as their only clinical finding. Significant risk factors associated with aortic thrombosis were calcium in the umbilical-artery-catheter infusate, placement of the catheter for any length of time above the renal arteries, and low birth weight. Physical examination did not reliably predict aortic thrombosis, and asymptomatic thrombosis was more frequent than previously recognized.