Catheterization of the aorta via the umbilical artery provides a convenient route for monitoring arterial blood pressure, for obtaining blood specimens for measurement of blood gas tensions and chemistries, and for the infusion of fluids and pharmacologic preparations in sick newborn infants. Use of this technique may be accompanied by a number of complications of which thrombotic phenomena are the most common. Twenty-three of 98 (24%) newborn infants undergoing umbilical artery catheterization were found to have thrombotic complications determined by aortography. No correlation was present between the duration of time that the umbilical artery catheters were in place and the occurrence of thrombotic complications. From paired aortographic or aortographic and autopsy studies in 24 patients, it was concluded that if a thrombotic complication did not occur early, none was likely to occur subsequently. One patient was considered to have died as a direct result of a thrombotic complication. Aortography is a safe, simple, and reliable technique for the early detection of thrombotic complications of umbilical artery catheters. Umbilical artery catheterization is not without risk and careful selection of patients for this procedure is indicated.