OBJECTIVE: To describe the maturation of physiologic milestones in preterm infants born between 24 and 32 weeks’ gestational age. METHODS: We abstracted daily physiologic maturity information on 865 infants born at or =32 weeks’ gestation in the Northern California Kaiser Permanente Medical Care Program between 1998 and 2001. Data included ventilator and incubator settings, body temperature, apnea and bradycardia spells, use of methylxanthines, feeding method, and requirements for intravenous fluids. Multivariable quantile regression models identified risk factors associated with longer postmenstrual age to achieve various physiologic milestones, including time to full oral feeding, time to wean from supplemental heat, the last day with an apnea or bradycardia episode, the last day on methylxanthine medications, and the last day on supplemental oxygen. RESULTS: Most milestones were achieved between 34 and 36 weeks’ postmenstrual age, although there were wide variations between patients. In most cases, feeding and oxygen milestones were achieved last. For each milestone there was a monotonic relationship between birth gestational age and the median achievement postmenstrual age. However, bronchopulmonary dysplasia and necrotizing enterocolitis strongly influenced these results in infants of younger gestational age. CONCLUSIONS: This study provides epidemiologic data describing the achievement of basic physiologic milestones that influence the discharge of a premature infant. This work serves as an additional contribution in the development of algorithms to monitor the progress of neonates through their initial hospitalization and provides a reference population for future interventions to improve the physiologic maturation of prematurely born infants.