Background and objective: Cardiopulmonary resuscitation (CPR) with adequate chest compression depth appears to improve first shock success in cardiac arrest. We evaluate the effect of simplification of chest compression instructions on compression depth in dispatcher-assisted CPR protocol. Methods: Data from two randomized, double-blinded, controlled trials with identical methodology were combined to obtain 332 records for this analysis. Subjects were randomized to either modified Medical Priority Dispatch System (MPDS) v11.2 protocol or a new simplified protocol. The main difference between the protocols was the instruction to "push as hard as you can" in the simplified protocol, compared to "push down firmly 2 in. (5 cm)" in MPDS. Data were recorded via a Laerdal® ResusciAnne® SkillReporter™ manikin. Primary outcome measures included: chest compression depth, proportion of compressions without error, with adequate depth and with total release. Results: Instructions to "push as hard as you can", compared to "push down firmly 2 in. (5 cm)", resulted in improved chest compression depth (36.4 mm vs. 29.7 mm, p < 0.0001), and improved median proportion of chest compressions done to the correct depth (32% vs. <1%, p < 0.0001). No significant difference in median proportion of compressions with total release (100% for both) and average compression rate (99.7 min-1 vs. 97.5 min-1, p < 0.56) was found. Conclusions: Modifying dispatcher-assisted CPR instructions by changing "push down firmly 2 in. (5 cm)" to "push as hard as you can" achieved improvement in chest compression depth at no cost to total release or average chest compression rate. © 2008 Elsevier Ireland Ltd. All rights reserved.