OBJECTIVE: To determine the rate of low-birthweight (LBW) births and the association of LBW with utilization and healthcare charges in a managed care organization. DESIGN: Observational study of computerized and medical record data. PATIENTS AND METHODS: We assessed the rate of LBW (weight < 2500 g) for singleton infants born during 1993 and 1995 at 2 hospitals (1993, N = 3212; 1995, N = 3073). For a subset of infants born during 1995 (n = 1273), we examined differences in utilization and medical charges, by birthweight category (moderately LBW [MLBW; 1500 to 2499 g] vs normal birthweight [NBW]), at 1 year postdischarge. RESULTS: In both 1993 and 1995, 3% of singleton infants were LBW infants, and 2% to 3% were macrosomic (> or = 4500 g). Complete data for analyses of utilization and healthcare charges were available on 1273 infants who were enrolled for the entire postdischarge year. The use of outpatient, emergency department (ED), and subspecialty care by MLBW infants and by NBW infants was similar. However, MLBW was associated with an increased rate of rehospitalization during the first year of birth (P < .01). MLBW infants' medical care charges were 46% higher than those of NBW infants (P = .0125). CONCLUSIONS: MLBW infants and NBW infants had similar outpatient and ED service use during the first year after hospital discharge. Excess charges incurred by MLBW infants were primarily due to higher rates of rehospitalization. Of the 38 admissions, 21 were related to infection or fear of infection, and 4 were due to congenital malformations.