We previously reported the early natural history of aortic thrombosis occurring after umbilical artery catheterization in 21 neonates. Ten of those neonates were reevaluated at 36-42 months of age for evidence of hypertension, renal abnormalities, and leg-growth disturbances. They were compared with an age-matched control group of seven infants. Blood pressures were greater than the 95th percentile in three infants and between the 50th and the 95th percentile in six of the 10 infants. Height was less than the fifth percentile for age in four infants with aortic thrombosis. One child had a 1.0-cm discrepancy in leg-length measurements, and seven of nine patients exhibited a 0.5-2.0 cm discrepancy between legs in either thigh or calf circumference. Sonography showed no evidence of residual clot in the aorta or renal vessels. Doppler flow was normal in all cases. Despite resolution of neonatal aortic thrombosis, complications resulting in renovascular hypertension (three of 10 patients) and leg-growth abnormalities (eight of nine) can occur. Hypertension (one of seven) and leg-growth discrepancy (four of seven) were less frequent in the 3-year follow-up of the seven matched control infants. We suggest that infants with known aortic thrombosis receive long-term follow-up to detect these potential problems.