The most important dilemma in the diagnosis of ventilator-associated pneumonia (VAP) based on only clinical findings is overdiagnosis. The aim of the study is to prospectively evaluate the Clinical Pulmonary Infection Score (CPIS) in relation to VAP diagnosis. Design. Prospective, in a cohort of mechanically ventilated patients. Setting. The intensive care unit of a university hospital. Patients. Fifty patients, on mechanical ventilation therapy for more than 48 hours, suspected of having VAP were enrolled in the study and bacteriologic confirmation was done by bronchoalveolar lavage (BAL) culture. Interventions. Bronchoscopy with BAL fluid culture after establishing a clinical suspicion of VAP in patients having no prior antibiotic therapy or no change in current antibiotic therapy within last three days before BAL. CPIS scores during diagnosis were 6±2 (3-9) (median±QR, maximum-minimum) and it was 7±2 (2-9) at the 72nd hour, in 41 cases with a diagnosis of VAP. In cases with no diagnosis of VAP, the CPIS scores were found to be 6±2 (4-8) and 5±3 (2-7), respectively. There was no significant difference between the VAP group and the non-VAP group at diagnosis, but was significant at 72nd hour (respectively, p=0.551 and p=0.025). CPIS scores during diagnosis were 6±3 (4-8) (median± QR, maximum-minimum) and 7±4 (2-8) at the 72nd hour, in 14 cases with a diagnosis of early-onset VAP. In cases with a diagnosis of late-onset VAP, the CPIS scores were found to be 6±2 (3-9) and 7±2 (3-9), respectively. There was no significant difference between the early-onset VAP group and the late-onset VAP group. In conclusion, the CPIS results should be evaluated carefully in the clinical setting during the diagnosis