Objective: To determine the effect of the strength of applied vacuum on the flow rate and yield of breastmilk using an electric breast pump.Study Design: Twenty-one breastfeeding mothers and two expressing mothers expressed their breastmilk for 15 minutes using an electric breast pump set at their own maximum comfortable vacuum, and at one to three softer vacuums. Milk yield and flow rate were measured.Results: At the maximum comfortable vacuum (−190.7 ± 8.8 mm Hg) 4.3 ± 0.4 milk ejections occurred during 15 minutes of expression and yielded 118.5 ± 11.4 mL of milk (65.5 ± 4.1% of the available milk). Softer vacuums yielded less milk volume (p <0.05) and less of the available milk (p <0.01). Milk flow rate was greater during the first milk ejection than the third or subsequent milk ejections (p <0.001). Cream content of the milk was highest after expressing for 15 minutes using the mother's maximum comfortable vacuum.Conclusions: Use of the mother's maximum comfortable vacuum enhances milk flow rate and milk yield. The cream content of the milk at the end of the expression period was an indicator of how effectively the breast had been drained.