Seventeen babies who developed hypertension (systolic BP greater than 113 mm Hg) after discharge from a neonatal intensive care unit were followed for a period ranging from 6 to 42 months. Five of the babies had an apparent cause for hypertension--one each had coarctation of the aorta, neuroblastoma, renal artery thrombosis, and two ureteropelvic junction obstruction. Umbilical artery catheters had been used in two additional babies. Ten of the babies had no obvious etiology for their hypertension. No prenatal, obstetrical or postnatal factor could be determined that predisposed these babies to hypertension. Sixteen of the babies were treated, 4 by surgical techniques and 12 by drugs--propranolol with or without chlorothiazide. All children treated medically were able to discontinue antihypertensives by 24 months of age. We conclude that follow-up of all babies discharged from the neonatal intensive care unit should include careful monitoring of blood pressure; that such infants may develop hypertension of unknown cause and that hypertension in this patient population is responsive to medication. Additional study is required to determine if babies discharged from neonatal intensive care units are at high risk for developing hypertension and to determine the natural history, prognosis and optimum treatment for the babies.