Background: Hospital-acquired pneumonia (HAP) prevalence is related to patients' population and diagnostic methods. HAP incidence is reported in different studies from 9% to 46%. The current study aimed at evaluating the etiology of HAP in patients admitted to intensive care units (ICUs). Methods:The patients admitted to ICU of Imam Reza hospital (a tertiary care and teaching hospital in Mashhad, Northeast of Iran) were evaluated for HAP based on the following criteria: fever, leukocytosis, purulent discharge, new radiologic findings, changes in O2 saturation, and ventilator setting. Blood and endotracheal aspirates cultures were performed for all patients. Demographic characteristics were recorded in a checklist. Results: Among the 88 adult patients enrolled in the current study, the most frequent radiologic finding was patchy infiltration in