A prospective study to detect renal artery thrombosis by radionuclide renal scintigraphy in newborn infants who underwent umbilical arterial catheterization over a one year period was done: 62 babies were catheterized, 92% were preterm and 85% had severe respiratory distress syndrome that required mechanical ventilation, 25/62 (40.3%) survived and in all of them Tc-99m DMSA scans were taken at a median of 5 days after withdrawal of the umbilical artery catheters (range 9 h to 29 days). A baby with renovascular hypertension had a DMSA scan which showed segmental vascular defect in one kidney. Another patient had left iliac artery thrombosis and two others showed evidence of transient vasospasm. Death occurred in 37/62 (59.7%), 92% of whom underwent autopsy studies which showed aortoiliac thrombosis in 8.8%, all of them without clinical symptoms. Other 12 newborn infants who died without previous umbilical artery catheterization had no evidence of thrombosis at autopsy.