Very low birth weights (VLBW) remain the major factor contributing to neonatal mortality and morbidity. The development of Neonatal Intensive Care Units (NICU) has improved the outcome for the VLBW infants. However, outborn VLBW infants may have different outcomes, and different medical costs than those born intramurally. This study compared the mortality, morbidity and costs of inborn and outborn VLBW infants cared in the NICU of a tertiary care center. A total of 176 VLBW infants (inborn 83, outborn 93) were examined over the three years period June 1990 to May 1993. The birth weights (1131 +/- 244 g vs 1133 +/- 255 g) and gestational ages (29.0 +/- 4.0 wk vs 28.9 +/- 3.0 wk) were not different between the two groups. However, the age of admission to our wards was significantly different between the inborn infants (5.0 +/- 3.2 hr.) and outborn infants (53.6 +/- 26.8 hr.). There was no difference in mortality rates between the outborn infants (35.7%) and the inborn infants (32.9%), nor in the incidence of intraventricular hemorrhage, respiratory distress syndrome, sepsis, necrotizing enterocolitis, retinopathy of prematurity or abnormal auditory brainstem response. However the incidence of patent ductus arteriosus and chronic lung disease of the outborn infants was higher than those of the inborn (47% vs 32%, 51% vs 29% respectively). The mean duration of hospitalization and cost seemed to be longer and higher in the outborn VLBW infants. It was concluded that outborn VLBW infants have higher rates of morbidity, longer hospitalization and cost more than inborn infants.(ABSTRACT TRUNCATED AT 250 WORDS)