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Introduction: The development of ventilator-associated pneumonia (VAP) is sometimes used as an index of the quality of care. However, the criteria used to diagnose VAP are variable. Using more strict criteria to diagnose VAP may result in a lower reported incidence of VAP and misleadingly suggest better quality of care. Methods: We included all adult patients who were treated with mechanical ventilation for more than 48 hours over a 7-month period (January-July 2012), and who had no lung infection during the first 48 hours of ventilation. We applied 89 algorithms composed of different criteria, including respiratory deterioration, inflammatory response, purulent tracheal secretions, abnormal chest radiography, and positive microbiologic findings. Results: Of 1806 patients admitted during the study period, 144 (8%) were treated with mechanical ventilation for more than 48 hours; 91 of these patients had no evidence of lung infection during the first 48 hours of mechanical ventilation. The application of the 89 algorithms resulted in a highly variable incidence of VAP, ranging from 0 to 44%. The mortality rate increased with increasing strictness of the criteria used, from 50 to 80% Conclusions: Applying different diagnostic criteria in a given patient population can result in a wide variation in the apparent incidence of VAP, and even eliminate it completely. The inverse correlation between the incidence of VAP and mortality, suggests that stricter criteria are more specific for VAP.