From July 1997 to June 1998, 25 preterm infants (birth weight < 1800 g) were included in a prospective study to compare the clinical effects of breast- and bottle-feeding. Oxygen saturation, heart rate, respiratory rate, and body temperature were recorded every minute for 20 minutes during feeding periods. Eighty pairs of breast- and bottle-feeding sessions were observed at the chronological age of 9.3 +/- 4.3 (range = 2.1-25.3) weeks. Oxygen saturation and body temperature of the preterm infants were significantly higher when they were directly breastfed. There were 2 episodes of apnea (breath pause more than 20 seconds) and 20 episodes of oxygen desaturation (PaO2 < 90%) during bottle-feeding and none during breastfeeding. We conclude that breastfeeding is a more physiological feeding method for the preterm infant and bottle-feeding may be more stressful.