In the United States debate continues about the necessity of ventilation during CPR because of fear of contracting infectious diseases. Three questions will be considered in this article. First, is ventilation necessary for the treatment of cardiac arrest? Second, is mouth-to-mouth ventilation any better than no ventilation at all? Third, are other techniques of ventilation as effective or more effective than mouth-to-mouth ventilation during basic life support CPR? Although research is still inconclusive with regard to the need for ventilation during CPR, recent findings have clarified the effect of ventilation during low blood flow states and how ventilation influences resuscitation. Ventilation affects oxygenation, carbon dioxide elimination, and pH during times of low rates of blood flow. Ventilation may be unnecessary during the first few minutes of CPR. Under conditions of prolonged, untreated cardiac arrest, ventilation during CPR affects return of spontaneous circulation. Isolated hypoxemia and hypercarbia independently have adverse effects on survival of cardiac arrest. Because ventilation with exhaled gas contains as much as 4% CO₂ and less oxygen than air, it may have adverse effects during CPR. Spontaneous gasping may provide sufficient ventilation during CPR. Chest compression alone provides some pulmonary ventilation and gas exchange. Active chest compression-decompression may improve gas exchange better than does standard chest compression. Other forms of manual ventilation may also have a role in CPR. [Idris AH: Reassessing the need for ventilation during CPR. Ann Emerg Med May 1996;27:569-575.]