The authors compared breastfeeding and expression characteristics in 30 mothers of exclusively breastfeeding, healthy term infants. Mean (± SD)volume per breastfeed from one breast was 71.8 ± 26.3 mL, and mean duration per breastfeed for one breastwas 16.6 ± 10.5 minutes. Mean volume of milk expressed in 5 minutes from one breast was 60.6 ± 39.0 mL and corresponded to the expression of 99.4 ± 82.6% of the milk stored in the breast. The rate of milk expression differed greatly between mothers ( P = .0001)b ut remained constant for the first 2.5 minutes before decreasing with time ( P = .0001). These results show the mean breastfeed volumewas similar to the volume of milk expressed in a 5-minute period. Furthermore, this study is the first to establish protocols that allow for the objective determination of breast pump efficacy.