The neonatal period is a time of extensive hemodynamic changes. It is expected that these changes are most prominent in premature infants during the first week of life. The aim of this study was to examine arterial blood pressure (BP) measured by an oscillometric device in the first month of life in a stable premature population admitted to our neonatal intensive care unit (NICU), and to evaluate the influence of gestational age, postnatal age, birth weight, gender, and sleep state on BP. This prospective study was conducted over 27 months. The study population consisted of 373 hemodynamically stable infants (292 preterm and 81 full-term infants). Overall 12,552 BP measurements were carried out using a non-invasive oscillometric blood pressure monitor. Both systolic and diastolic blood pressure progressively increased during the first month of life. BP increased more rapidly in preterm infants than in full-term infants, and was higher in groups with higher birth weight. Multiple regression analysis showed that mean BP during the first week and on the 30th day increased with gestational age, and also that it was higher in the awake than in the sleep state.