Recent clinical studies reporting the high frequency of inadequate chest compression depth (<38mm) during CPR, have prompted the question if adult human chest characteristics render it difficult to attain the recommended compression depth in certain patients.Using a specially designed monitor/defibrillator equipped with a sternal pad fitted with an accelerometer and a pressure sensor, compression force and depth was measured during CPR in 91 adult out-of-hospital cardiac arrest patients.There was a strong non-linear relationship between the force of compression and depth achieved. Mean applied force for all patients was 30.3±8.2kg and mean absolute compression depth 42±8mm. For 87 of 91 patients 38mm compression depth was obtained with less than 50kg. Stiffer chests were compressed more forcefully than softer chests (p<0.001), but softer chests were compressed more deeply than stiffer chests (p=0.001). The force needed to reach 38mm compression depth (F 38 ) and mean compression force were higher for males than for females: 29.8±14.5kg versus 22.5±10.2kg (p<0.02), and 32.0±8.3kg versus 27.0±7.0kg (p<0.01), respectively. There was no significant variation in F 38 or compression depth with age, but a significant 1.5kg mean decrease in applied force for each 10 years increase in age (p<0.05). Chest stiffness decreased significantly (p<0.0001) with an increasing number of compressions performed. Average residual force during decompression was 1.7±1.0kg, corresponding to an average residual depth of 3±2mm.In most out-of-hospital cardiac arrest victims adequate chest compression depth can be achieved by a force <50kg, indicating that an average sized and fit rescuer should be able to perform effective CPR in most adult patients.