Objective To determine the effect of closed tracheal suction system versus open tracheal suction system on the rate of ventilator-associated pneumonia in adults. Methods We searched The Cochrane Library (Issue 1, 2007), PubMed (1966 to 2006) and CBM (1980 to 2007), and also hand searched relevant journals. Randomized controlled trials involving closed tracheal suction system versus open tracheal suction system for ventilator-associated pneumonia in adults were included. Data were extracted and the quality of trials was critical assessed by two reviewers independently. The Cochrane Collaboration’s RevMan 4.2.8 software was used for data analyses. Result Five randomized controlled trials involving 739 patients were included. Results of meta-analyses showed that compared to open tracheal suction system, closed tracheal suction system did not increase the rate of ventilator-associated pneumonia (RR 0.83, 95%CI 0.50 to 1.37) or case fatality (RR 1.05, 95%CI 0.85 to 1.31). No significant differences were observed between open tracheal suction system and closed tracheal suction system in the total number of bacteria (RR 0.83, 95%CI 0.50 to 1.37), the number of SPP colony (RR 2.87, 95%CI 0.94 to 8.74) and the number of PSE colony (RR 1.46, 95%CI 0.76 to 2.77). There was no significant difference between the two groups in the duration of ventilation and length of hospital stay. Conclusion Open or closed tracheal suction systems have similar effects on the rate of ventilator-associated pneumonia, case fatality, the number of SPP and PSE colonies, duration of ventilation and length of hospital stay. However, due to the differences in interventions and statistical power among studies included in this systematic review, further studies are needed to determine the effect of closed or open tracheal suction systems on these outcomes.