A constellation of adverse effects and complications may be associated with mechanical ventilation, although in many instances the causal role of the ventilator itself has not been established. Complications occur with greater frequency than is generally appreciated, and tend to be under-reported in the medical literature. Among the potential adverse physiologic effects of positive-pressure ventilation are decreased cardiac output, unintended respiratory alkalosis, increased intracranial pressure, gastric distension, and impairment of hepatic and renal function. Failure of the ventilator to cycle, of safety alarms to function properly, and of inspired gas to be properly heated or humidified are examples of equipment-related complications. Perhaps most feared among medical complications occurring during mechanical ventilation are pneumothorax, bronchopleural fistula, and the development of nosocomial pneumonia; these entities may owe as much to the impairment of host defenses and normal tissue integrity as to the presence of the ventilator per se. Finally, a variety of avoidable "misadventures," due primarily to lapses of understanding and communication among the physicians, nurses, and respiratory care practitioners managing the ventilated patient, can adversely affect comfort, morbidity, and ultimate outcome.