There are few blood pressure (BP) data reported for premature and term newborn infants after 24 hours of age. To determine BP levels and BP trends in a representative population of infants admitted to neonatal intensive care units (NICUs), this study was conducted in 14 NICUs in the greater Philadelphia area. All infants admitted to the 14 NICUs during a 3-month period were entered into the study. BP data, along with data on clinical conditions and therapeutic interventions (independent variables), were prospectively collected by a uniform protocol. Systolic BP (SBP) and diastolic BP (DBP) were measured indirectly by oscillometry and recorded every 8 hours. Data from 608 infants followed up for 1 to 99 days after delivery generated 9911 infant-day records and 24,052 individual BP measurements. On day 1, birth weight and gestational age were strong correlates of SBP (r = 0.68, p < 0.0001 and r = 0.66, p < 0.0001, respectively) and DBP (r = 0.48, p < 0.0001 and r = 0.47, p < 0.001, respectively). During the first 5 days of life there was a progressive rise in SBP (2.23 to 2.67 mm Hg/day) and DBP (1.58 to 2.02 mm Hg/day) regardless of gestational age or weight at birth. After day 5 there was a more gradual increment in the daily SBP (0.24 to 0.27 mm Hg/day) and DBP (0 to 0.15 mm Hg/day). Stepwise linear multiple regressions were used to examine the multiple correlations among the independent variables and to build a regression model for BP. Gestational age and day of life emerged in the first two steps of the multiple regression analysis (multiple R = 0.463 and 0.655, respectively; p < 0.0001 for both). The multiple R values for day of life and gestational age were virtually identical to that for postconceptional age (day of life + gestational age at birth). Although other common diagnosis and treatment variables contributed a small amount to the total variance in BP, postconceptional age was the primary determinant of BP in this population of infants.