Self-inflating manual resuscitators (SIMRs) can mislead caregivers because the bag, unlike a Mapleson-type device, reinflates even without patient exhalation. We added a whistle as an audible indicator to the exhalation port of a SIMR. In randomized order, each participant provided two sets of breaths via mask ventilation with a SIMR, one with and one without audible feedback, to a Human Patient Simulator modified to log lung volume changes. The last three breaths in each set were used to compare average tidal volume (Vt) under both conditions. Eighty-seven advanced cardiac life support trainees (54 males, 33 females) with clinical experience averaging 6.4 +/- 9.4 yr were recruited. Average Vt delivered with the standard SIMR was 486 +/- 166 mL and 624 +/- 96 mL with the modified SIMR. Average Vt delivered by a modified SIMR was significantly larger by 40% when it followed standard SIMR use and 19% when using the modified SIMR first. Use of a SIMR with an audible indicator of exhalation significantly (P < 0.001) increased mask ventilation of a patient simulator, suggesting that mask ventilation of a patient with a SIMR may also be increased by objective, real-time feedback of exhaled Vt.