The aim of this study was to compare the effect of closed versus open endotracheal suction systems on the development of ventilator-associated pneumonia (VAP). A prospective, randomized, controlled trial was performed in a medical intensive care unit (MICU) of a university hospital in patients who received mechanical ventilation for more than 48 h. Patients were randomized to receive endotracheal suction with either closed catheters (closed suction group; N-41) or single-use catheters (open suction group; N=37). Cultures were taken from the ventilator tubing of 42 patients to determine the rate of colonization. There was no difference between the groups in terms of the frequency of development of VAP, mortality in the MICU, length of MICU stay and duration of mechanical ventilation. Thirteen patients in the open suction group and 16 patients in the closed suction group became colonized (P=0.14). The colonization rates by Acinetobacter spp. and Pseudomonas aeruginosa were more frequent in the closed suction group than in the open suction group (P<0.01 and P=0.04, respectively). In conclusion, closed endotracheal suction resulted in increased colonization rates of ventilator tubing with multi drug-resistant micro-organisms but did not increase the development of VAP and MICU outcome compared with open endotracheal suction.