OBJECTIVES: Despite efforts to decrease the incidence of low birth weight (LBW) and pre-term infants, rates for these conditions have continued to rise in recent years. The objective of this study was to determine the recent prevalence of and burden associated with hospitalizations among LBW and pre-term infants in the United States (US).   METHODS: This study used data from the 2008 Healthcare Cost and Utilization Project Nationwide Inpatient Sample. Hospital stays were selected for inclusion if the patient was aged ≤1 year old. Stays were broken into three categories: LBW/pre-term stays (any diagnosis with an ICD-9-CM code of 764.xx, 765.xx, and V21.3x), uncomplicated newborn stays (primary diagnosis with an ICD-9-CM code between V30 and V39.2), and all other infant stays. LBW/pre-term stays were stratified by infants weighing less than or greater than 2,500 grams. Study measures were weighted and included demographics, hospital characteristics, length of stay (LOS), and costs.     RESULTS: In 2008 there were 499,473 stays for LBW/pre-term infants, representing 10% of all infant stays. The average LOS for LBW/pre-term infants was 11.9 days, versus 2.3 days among infants with an uncomplicated newborn stay, and 4.2 days among all other infant stays. LBW/pre-term infant stays resulted in costs of more than $9.7 billion, or approximately 45% of all costs for all infant stays, and nearly 1.7 times the costs of uncomplicated newborn stays. Patients with a birth weight less than 2,500 grams had costs that were approximately 3.9 times greater than costs for patients with a birth weight greater than 2,500 grams (mean [SE] $23,382 [$1,220] versus $5,951 [$350] among patients with a birth weight less than versus greater than 2,500 grams, respectively).  CONCLUSIONS: While LBW/pre-term infant stays represent a small percentage of all infant hospitalizations, they accrue almost half of all inpatient costs among infants.