• The aim of the present study was to elucidate the role of hole size and thickness in determining milk flow through nipple units during bottle feeding. Commonly used standard nipple units (SMA single-hole, Enfamil single-hole, and Twist-on) for term and preterm infants, as well as Nuk-type nipple units (SMA Nuk, Enfamil Natural, and Nuk) were tested. The size of the nipple hole and wall thickness were determined for each nipple unit. Airflow was measured by forcing pressurized air through the feed hole. Simulated sucks were used to measure the milk flow. A marked variability in airflow and milk flow was observed within and among the various types of nipple units studied. Within each type of nipple unit, both milk flow and airflow measurements correlated well with hole size. The thickness of the nipple units contributed minimally to the observed variability. We conclude that differences in hole size primarily account for the observed variability in milk flow. This finding may be clinically important in that rapid milk flow can lead to apnea and bradycardia in some preterm infants. The above observations imply that design changes are necessary to reduce the variability of milk flow within each nipple type. Moreover, milk-flow measurements made using a simple mechanical system and airflow measurements used by the industry are equally sensitive to evaluate nipple flow.