To determine the fractional delivered oxygen concentrations (FDO2) and the proper functioning of patient valves in adult resuscitation bags, we studied the Laerdal, the PMR 1, the PMR 2, and the Vitalograph resuscitators at various oxygen flowrates and ventilation patterns, with and without reservoirs. The Vitalograph, with new or old reservoir, and the PMR 1 cannot deliver high oxygen concentrations (FDO2 > 0.90) and should not be used when high FDO2 is important. The PMR 2 and the Laerdal cannot deliver high oxygen concentrations without reservoirs attached; with reservoirs attached the Laerdal achieves FDO2 > 0.98 at a flowrate of 10 l/min, and the PMR 2 achieves FDO2 > 0.90 at a flowrate of 15 l/min.