Stimulated blood flow was measured in the legs of 28 infants who had undergone umbilical arterial catheterization in the neonatal period. Catheter tips were positioned in the region of the aortic bifurcation, and only an isotonic saline/dextrose solution was continuously pumped through the catheter. The catheters were in place for an average of 58.3 hours (range, 4 to 144), and the infants were studied between 29 and 135 days of age (mean, 67 days). Blood flow in both legs was measured simultaneously by venous occlusion plethysmography using a mercury-in-rubber strain gauge. Analysis of peak stimulated blood flow in each leg and simultaneous flow in the opposite leg showed no difference between flow in the leg whose iliac artery had been catheterized in the neonatal period and flow in the opposite leg (paired t-test equal 0.17; P greater.50). No chronic, subclinical flow deficiencies of umbilical arterial catheterization were demonstrable. We see no new reason to curtail the judicious use of the umbilical artery catheter.