OBJECTIVE This study attempted to identify which is the more effective suction system. The objective was to compare open versus closed suction systems according to a systematic review. METHODS A search of scientific literature was conducted in MedLine, LILACS and Cochrane between 1997 and August 2007 using the key words: endotracheal suction and closed suction. Included were articles that compared the open and closed suction systems used in adult humans and that were randomized and controlled trials. RESULTS From the 78 articles identified, only 15 were accepted and described in this review. Nine compared incidence of ventilator-associated pneumonia, six compared oxygen saturation, four compared blood pressure and heart rate, three compared pulmonary volumes, two compared secretion removal and four compared costs. No difference was found in these variables compared: incidence of ventilator associated pneumonia, mortality, intensive care unit length of stay, duration of mechanical ventilation, PaCO2, PaO2, mean blood pressure, heart rate and secretion removal. However, there were always SpO2 and pulmonary volume decreases when using the open suction system; and costs were lower in most of the studies that used the closed suction system. CONCLUSIONS Closed suction system seems to increase the risk of colonization, but has the advantage of not reducing the pulmonary volumes and not entailing a drop of saturation, especially in patients with severe respiratory failure and in the use of higher levels of positive end expiratory pressure.