The authors compared milk expression using 5 experimental vacuum patterns and a commercially available vacuum pattern ranging in cycle times (20 to 78 cycles/min) and vacuum curve dynamics in 30 mothers using an experimental, software-controlled electric breast pump. The volume of milk removed over 5 minutes differed (P = .0072) between patterns (range = 62.8 +/- 6.6 mL to 47.2 +/- 5.1 mL). However, there was no difference in the percentage of available milk removed (range = 99.4% +/- 15.1% to 70.6% +/- 8.6%). The rate of milk removal differed between patterns at both the beginning (1 minute) and end (1.5 minutes) of the expression period (P < .05). Peak vacuum chosen differed between patterns (P = .0085) but was not related to either the volume or percentage of available milk expressed. Breastfeeding characteristics did not differ between poor and successful expressers. These results show that breast milk expression by an electric breast pump can be influenced by the vacuum pattern.